build: wire prefractal into CI and the README feature list

The prefractal module landed on main in parallel with the branches that
established the current module conventions, so it never picked up the
integration points every other experimental module has. 01379624 covered
the CMake experimental gate and EXTRA_DIST; these two are what remained.

The CI gap is the more consequential of the pair. ci/ci.sh had no
PREFRACTAL plumbing at all, so no CI job has ever passed
--enable-module-prefractal -- the flag was reachable only by configuring
by hand, and the module has been built and tested exclusively outside CI
since it landed.

- ci/ci.sh: PREFRACTAL joins FROST, CHILLDKG and ICEBERG in the
  reproduction header's variable list, and configure gains
  --enable-module-prefractal="$PREFRACTAL" after the iceberg line.
  Unlike 2473c768 there is no bench-step counterpart: prefractal ships
  neither a bench binary nor an example program, so the BENCH block
  needs nothing.

- .github/workflows/ci.yml: a PREFRACTAL: 'no' default alongside the
  other module defaults, then PREFRACTAL: 'yes' in the 21 places that
  already enable FROST (11 inline matrix entries and 10 job-level env
  blocks). The default is not cosmetic -- ci.sh runs under set -eux, so
  an undefined PREFRACTAL would abort every job at the configure step,
  including the ones that build no modules at all.

FROST-enabled jobs are the right target because enabling prefractal
implies frost (configure.ac:539, mirrored in src/CMakeLists.txt:90):
adding PREFRACTAL to a job that sets FROST: 'no' would silently turn
frost on and change what that job covers. Every job that already builds
frost also enables musig, schnorrsig, extrakeys and experimental, which
is the remainder of prefractal's dependency chain, so no job needed any
other variable adjusted.

This is a slightly wider set than CHILLDKG and ICEBERG cover. The
x86_64-debian matrix entry at line 117 sets FROST: 'yes' without
CHILLDKG or ICEBERG, and 2473c768 deliberately left it alone; it gets
PREFRACTAL here, on the rule above. The consequence is that no job now
builds frost without prefractal. If that standalone-frost combination is
worth preserving, that one entry is the place to drop it.

README.md gains the doc/prefractal.md link beside the frost, chilldkg
and iceberg entries. This is the exact inverse of the bug 42f827a7
fixed: there the documents were linked from README.md but missing from
EXTRA_DIST, so the tarball's README pointed at files it did not carry.
Here doc/prefractal.md has been in EXTRA_DIST since 01379624 but nothing
referenced it, so it shipped unreferenced. All four module documents
this fork adds are now both linked and distributed.

Verified with ./autogen.sh, ./configure --enable-experimental
--enable-module-prefractal and make -j: all exit 0 with no warnings, and
the configure summary shows the implication chain resolving, with
prefractal = yes pulling in frost = yes and musig = yes. nm confirms
run_prefractal_api_test is linked into ./tests, and the suite passes.
make dist succeeds and the tarball carries doc/prefractal.md next to
doc/iceberg.md, src/modules/frost/frost.md and
src/modules/chilldkg/chilldkg.md. ci/ci.sh passes sh -n.

Verified programmatically for the workflow, as in 2473c768: the YAML
parses, 33 effective job contexts set FROST: 'yes', all 33 of them now
also set PREFRACTAL: 'yes', every one has MUSIG, SCHNORRSIG, EXTRAKEYS
and EXPERIMENTAL set to 'yes', and no context sets PREFRACTAL without
FROST.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -50,6 +50,7 @@ env:
   FROST: 'no'
   CHILLDKG: 'no'
   ICEBERG: 'no'
+  PREFRACTAL: 'no'
   ### test options
   SECP256K1_TEST_ITERS: 64
   BENCH: 'yes'
@@ -107,14 +108,14 @@ jobs:
       matrix:
         configuration:
           - env_vars: { WIDEMUL: 'int64',  RECOVERY: 'yes' }
-          - env_vars: { WIDEMUL: 'int64',                   ECDH: 'yes', EXTRAKEYS: 'yes', SCHNORRSIG: 'yes', MUSIG: 'yes', ELLSWIFT: 'yes', EXPERIMENTAL: 'yes', ECDSA_S2C: 'yes', RANGEPROOF: 'yes', SURJECTIONPROOF: 'yes', WHITELIST: 'yes', GENERATOR: 'yes', ECDSAADAPTOR: 'yes', BPPP: 'yes', SCHNORRSIG_HALFAGG: 'yes', FROST: 'yes', CHILLDKG: 'yes', ICEBERG: 'yes'}
+          - env_vars: { WIDEMUL: 'int64',                   ECDH: 'yes', EXTRAKEYS: 'yes', SCHNORRSIG: 'yes', MUSIG: 'yes', ELLSWIFT: 'yes', EXPERIMENTAL: 'yes', ECDSA_S2C: 'yes', RANGEPROOF: 'yes', SURJECTIONPROOF: 'yes', WHITELIST: 'yes', GENERATOR: 'yes', ECDSAADAPTOR: 'yes', BPPP: 'yes', SCHNORRSIG_HALFAGG: 'yes', FROST: 'yes', CHILLDKG: 'yes', ICEBERG: 'yes', PREFRACTAL: 'yes'}
           - env_vars: { WIDEMUL: 'int128' }
           - env_vars: { WIDEMUL: 'int128_struct',                                                                           ELLSWIFT: 'yes' }
           - env_vars: { WIDEMUL: 'int128', RECOVERY: 'yes',              EXTRAKEYS: 'yes', SCHNORRSIG: 'yes', MUSIG: 'yes', ELLSWIFT: 'yes' }
-          - env_vars: { WIDEMUL: 'int128',                  ECDH: 'yes', EXTRAKEYS: 'yes', SCHNORRSIG: 'yes', MUSIG: 'yes',                  EXPERIMENTAL: 'yes', ECDSA_S2C: 'yes', RANGEPROOF: 'yes', SURJECTIONPROOF: 'yes', WHITELIST: 'yes', GENERATOR: 'yes', ECDSAADAPTOR: 'yes', BPPP: 'yes', SCHNORRSIG_HALFAGG: 'yes', FROST: 'yes', CHILLDKG: 'yes', ICEBERG: 'yes'}
+          - env_vars: { WIDEMUL: 'int128',                  ECDH: 'yes', EXTRAKEYS: 'yes', SCHNORRSIG: 'yes', MUSIG: 'yes',                  EXPERIMENTAL: 'yes', ECDSA_S2C: 'yes', RANGEPROOF: 'yes', SURJECTIONPROOF: 'yes', WHITELIST: 'yes', GENERATOR: 'yes', ECDSAADAPTOR: 'yes', BPPP: 'yes', SCHNORRSIG_HALFAGG: 'yes', FROST: 'yes', CHILLDKG: 'yes', ICEBERG: 'yes', PREFRACTAL: 'yes'}
           - env_vars: { WIDEMUL: 'int128', ASM: 'x86_64',                                                                   ELLSWIFT: 'yes' }
-          - env_vars: {                    RECOVERY: 'yes',              EXTRAKEYS: 'yes', SCHNORRSIG: 'yes', MUSIG: 'yes',                  EXPERIMENTAL: 'yes', ECDSA_S2C: 'yes', RANGEPROOF: 'yes', SURJECTIONPROOF: 'yes', WHITELIST: 'yes', GENERATOR: 'yes', ECDSAADAPTOR: 'yes', BPPP: 'yes', SCHNORRSIG_HALFAGG: 'yes', FROST: 'yes'}
-          - env_vars: { CTIMETESTS: 'no',  RECOVERY: 'yes', ECDH: 'yes', EXTRAKEYS: 'yes', SCHNORRSIG: 'yes', MUSIG: 'yes',                  EXPERIMENTAL: 'yes', ECDSA_S2C: 'yes', RANGEPROOF: 'yes', SURJECTIONPROOF: 'yes', WHITELIST: 'yes', GENERATOR: 'yes', ECDSAADAPTOR: 'yes', BPPP: 'yes', SCHNORRSIG_HALFAGG: 'yes', FROST: 'yes', CHILLDKG: 'yes', ICEBERG: 'yes', CPPFLAGS: '-DVERIFY' }
+          - env_vars: {                    RECOVERY: 'yes',              EXTRAKEYS: 'yes', SCHNORRSIG: 'yes', MUSIG: 'yes',                  EXPERIMENTAL: 'yes', ECDSA_S2C: 'yes', RANGEPROOF: 'yes', SURJECTIONPROOF: 'yes', WHITELIST: 'yes', GENERATOR: 'yes', ECDSAADAPTOR: 'yes', BPPP: 'yes', SCHNORRSIG_HALFAGG: 'yes', FROST: 'yes', PREFRACTAL: 'yes'}
+          - env_vars: { CTIMETESTS: 'no',  RECOVERY: 'yes', ECDH: 'yes', EXTRAKEYS: 'yes', SCHNORRSIG: 'yes', MUSIG: 'yes',                  EXPERIMENTAL: 'yes', ECDSA_S2C: 'yes', RANGEPROOF: 'yes', SURJECTIONPROOF: 'yes', WHITELIST: 'yes', GENERATOR: 'yes', ECDSAADAPTOR: 'yes', BPPP: 'yes', SCHNORRSIG_HALFAGG: 'yes', FROST: 'yes', CHILLDKG: 'yes', ICEBERG: 'yes', PREFRACTAL: 'yes', CPPFLAGS: '-DVERIFY' }
           - env_vars: { BUILD: 'distcheck', WITH_VALGRIND: 'no', CTIMETESTS: 'no', BENCH: 'no' }
           - env_vars: { CPPFLAGS: '-DDETERMINISTIC' }
           - env_vars: { CFLAGS: '-O0', CTIMETESTS: 'no' }
@@ -183,6 +184,7 @@ jobs:
       FROST: 'yes'
       CHILLDKG: 'yes'
       ICEBERG: 'yes'
+      PREFRACTAL: 'yes'
       CC: ${{ matrix.cc }}
 
     steps:
@@ -223,6 +225,7 @@ jobs:
       FROST: 'yes'
       CHILLDKG: 'yes'
       ICEBERG: 'yes'
+      PREFRACTAL: 'yes'
       CTIMETESTS: 'no'
 
     steps:
@@ -265,6 +268,7 @@ jobs:
       FROST: 'yes'
       CHILLDKG: 'yes'
       ICEBERG: 'yes'
+      PREFRACTAL: 'yes'
       CTIMETESTS: 'no'
 
     steps:
@@ -298,6 +302,7 @@ jobs:
       FROST: 'yes'
       CHILLDKG: 'yes'
       ICEBERG: 'yes'
+      PREFRACTAL: 'yes'
       CTIMETESTS: 'no'
       CC: ${{ matrix.cc }}
 
@@ -350,6 +355,7 @@ jobs:
       FROST: 'yes'
       CHILLDKG: 'yes'
       ICEBERG: 'yes'
+      PREFRACTAL: 'yes'
       CTIMETESTS: 'no'
 
     steps:
@@ -407,6 +413,7 @@ jobs:
       FROST: 'yes'
       CHILLDKG: 'yes'
       ICEBERG: 'yes'
+      PREFRACTAL: 'yes'
       CTIMETESTS: 'no'
       SECP256K1_TEST_ITERS: 2
 
@@ -448,6 +455,7 @@ jobs:
       FROST: 'yes'
       CHILLDKG: 'yes'
       ICEBERG: 'yes'
+      PREFRACTAL: 'yes'
       CTIMETESTS: 'no'
       CFLAGS: '-fsanitize=undefined,address -g'
       UBSAN_OPTIONS: 'print_stacktrace=1:halt_on_error=1'
@@ -506,6 +514,7 @@ jobs:
       FROST: 'yes'
       CHILLDKG: 'yes'
       ICEBERG: 'yes'
+      PREFRACTAL: 'yes'
       CC: ${{ matrix.cc }}
       SECP256K1_TEST_ITERS: 32
       ASM: 'no'
@@ -543,6 +552,7 @@ jobs:
       FROST: 'yes'
       CHILLDKG: 'yes'
       ICEBERG: 'yes'
+      PREFRACTAL: 'yes'
       CTIMETESTS: 'no'
 
     strategy:
@@ -575,15 +585,15 @@ jobs:
       fail-fast: false
       matrix:
         env_vars:
-          - { WIDEMUL: 'int64',  RECOVERY: 'yes', ECDH: 'yes', EXTRAKEYS: 'yes', SCHNORRSIG: 'yes', MUSIG: 'yes', ELLSWIFT: 'yes', EXPERIMENTAL: 'yes', ECDSA_S2C: 'yes', RANGEPROOF: 'yes', SURJECTIONPROOF: 'yes', WHITELIST: 'yes', GENERATOR: 'yes', ECDSAADAPTOR: 'yes', BPPP: 'yes', SCHNORRSIG_HALFAGG: 'yes', FROST: 'yes', CHILLDKG: 'yes', ICEBERG: 'yes' }
+          - { WIDEMUL: 'int64',  RECOVERY: 'yes', ECDH: 'yes', EXTRAKEYS: 'yes', SCHNORRSIG: 'yes', MUSIG: 'yes', ELLSWIFT: 'yes', EXPERIMENTAL: 'yes', ECDSA_S2C: 'yes', RANGEPROOF: 'yes', SURJECTIONPROOF: 'yes', WHITELIST: 'yes', GENERATOR: 'yes', ECDSAADAPTOR: 'yes', BPPP: 'yes', SCHNORRSIG_HALFAGG: 'yes', FROST: 'yes', CHILLDKG: 'yes', ICEBERG: 'yes', PREFRACTAL: 'yes' }
           - { WIDEMUL: 'int128_struct', ECMULTGENKB: 2, ECMULTWINDOW: 4 }
-          - { WIDEMUL: 'int128',                  ECDH: 'yes', EXTRAKEYS: 'yes', SCHNORRSIG: 'yes', MUSIG: 'yes', ELLSWIFT: 'yes', EXPERIMENTAL: 'yes', ECDSA_S2C: 'yes', RANGEPROOF: 'yes', SURJECTIONPROOF: 'yes', WHITELIST: 'yes', GENERATOR: 'yes', ECDSAADAPTOR: 'yes', BPPP: 'yes', SCHNORRSIG_HALFAGG: 'yes', FROST: 'yes', CHILLDKG: 'yes', ICEBERG: 'yes' }
+          - { WIDEMUL: 'int128',                  ECDH: 'yes', EXTRAKEYS: 'yes', SCHNORRSIG: 'yes', MUSIG: 'yes', ELLSWIFT: 'yes', EXPERIMENTAL: 'yes', ECDSA_S2C: 'yes', RANGEPROOF: 'yes', SURJECTIONPROOF: 'yes', WHITELIST: 'yes', GENERATOR: 'yes', ECDSAADAPTOR: 'yes', BPPP: 'yes', SCHNORRSIG_HALFAGG: 'yes', FROST: 'yes', CHILLDKG: 'yes', ICEBERG: 'yes', PREFRACTAL: 'yes' }
           - { WIDEMUL: 'int128', RECOVERY: 'yes' }
-          - { WIDEMUL: 'int128', RECOVERY: 'yes', ECDH: 'yes', EXTRAKEYS: 'yes', SCHNORRSIG: 'yes', MUSIG: 'yes', ELLSWIFT: 'yes', EXPERIMENTAL: 'yes', ECDSA_S2C: 'yes', RANGEPROOF: 'yes', SURJECTIONPROOF: 'yes', WHITELIST: 'yes', GENERATOR: 'yes', ECDSAADAPTOR: 'yes', BPPP: 'yes', SCHNORRSIG_HALFAGG: 'yes', FROST: 'yes', CHILLDKG: 'yes', ICEBERG: 'yes' }
-          - { WIDEMUL: 'int128', RECOVERY: 'yes', ECDH: 'yes', EXTRAKEYS: 'yes', SCHNORRSIG: 'yes', MUSIG: 'yes', ELLSWIFT: 'yes', EXPERIMENTAL: 'yes', ECDSA_S2C: 'yes', RANGEPROOF: 'yes', SURJECTIONPROOF: 'yes', WHITELIST: 'yes', GENERATOR: 'yes', ECDSAADAPTOR: 'yes', BPPP: 'yes', SCHNORRSIG_HALFAGG: 'yes', FROST: 'yes', CHILLDKG: 'yes', ICEBERG: 'yes', CC: 'gcc' }
-          - { WIDEMUL: 'int128', RECOVERY: 'yes', ECDH: 'yes', EXTRAKEYS: 'yes', SCHNORRSIG: 'yes', MUSIG: 'yes', ELLSWIFT: 'yes', EXPERIMENTAL: 'yes', ECDSA_S2C: 'yes', RANGEPROOF: 'yes', SURJECTIONPROOF: 'yes', WHITELIST: 'yes', GENERATOR: 'yes', ECDSAADAPTOR: 'yes', BPPP: 'yes', SCHNORRSIG_HALFAGG: 'yes', FROST: 'yes', CHILLDKG: 'yes', ICEBERG: 'yes',  WRAPPER_CMD: 'valgrind --error-exitcode=42', SECP256K1_TEST_ITERS: 2 }
-          - { WIDEMUL: 'int128', RECOVERY: 'yes', ECDH: 'yes', EXTRAKEYS: 'yes', SCHNORRSIG: 'yes', MUSIG: 'yes', ELLSWIFT: 'yes', EXPERIMENTAL: 'yes', ECDSA_S2C: 'yes', RANGEPROOF: 'yes', SURJECTIONPROOF: 'yes', WHITELIST: 'yes', GENERATOR: 'yes', ECDSAADAPTOR: 'yes', BPPP: 'yes', SCHNORRSIG_HALFAGG: 'yes', FROST: 'yes', CHILLDKG: 'yes', ICEBERG: 'yes', CC: 'gcc', WRAPPER_CMD: 'valgrind --error-exitcode=42', SECP256K1_TEST_ITERS: 2 }
-          - { WIDEMUL: 'int128', RECOVERY: 'yes', ECDH: 'yes', EXTRAKEYS: 'yes', SCHNORRSIG: 'yes', MUSIG: 'yes', ELLSWIFT: 'yes', EXPERIMENTAL: 'yes', ECDSA_S2C: 'yes', RANGEPROOF: 'yes', SURJECTIONPROOF: 'yes', WHITELIST: 'yes', GENERATOR: 'yes', ECDSAADAPTOR: 'yes', BPPP: 'yes', SCHNORRSIG_HALFAGG: 'yes', FROST: 'yes', CHILLDKG: 'yes', ICEBERG: 'yes', CPPFLAGS: '-DVERIFY', CTIMETESTS: 'no' }
+          - { WIDEMUL: 'int128', RECOVERY: 'yes', ECDH: 'yes', EXTRAKEYS: 'yes', SCHNORRSIG: 'yes', MUSIG: 'yes', ELLSWIFT: 'yes', EXPERIMENTAL: 'yes', ECDSA_S2C: 'yes', RANGEPROOF: 'yes', SURJECTIONPROOF: 'yes', WHITELIST: 'yes', GENERATOR: 'yes', ECDSAADAPTOR: 'yes', BPPP: 'yes', SCHNORRSIG_HALFAGG: 'yes', FROST: 'yes', CHILLDKG: 'yes', ICEBERG: 'yes', PREFRACTAL: 'yes' }
+          - { WIDEMUL: 'int128', RECOVERY: 'yes', ECDH: 'yes', EXTRAKEYS: 'yes', SCHNORRSIG: 'yes', MUSIG: 'yes', ELLSWIFT: 'yes', EXPERIMENTAL: 'yes', ECDSA_S2C: 'yes', RANGEPROOF: 'yes', SURJECTIONPROOF: 'yes', WHITELIST: 'yes', GENERATOR: 'yes', ECDSAADAPTOR: 'yes', BPPP: 'yes', SCHNORRSIG_HALFAGG: 'yes', FROST: 'yes', CHILLDKG: 'yes', ICEBERG: 'yes', PREFRACTAL: 'yes', CC: 'gcc' }
+          - { WIDEMUL: 'int128', RECOVERY: 'yes', ECDH: 'yes', EXTRAKEYS: 'yes', SCHNORRSIG: 'yes', MUSIG: 'yes', ELLSWIFT: 'yes', EXPERIMENTAL: 'yes', ECDSA_S2C: 'yes', RANGEPROOF: 'yes', SURJECTIONPROOF: 'yes', WHITELIST: 'yes', GENERATOR: 'yes', ECDSAADAPTOR: 'yes', BPPP: 'yes', SCHNORRSIG_HALFAGG: 'yes', FROST: 'yes', CHILLDKG: 'yes', ICEBERG: 'yes', PREFRACTAL: 'yes',  WRAPPER_CMD: 'valgrind --error-exitcode=42', SECP256K1_TEST_ITERS: 2 }
+          - { WIDEMUL: 'int128', RECOVERY: 'yes', ECDH: 'yes', EXTRAKEYS: 'yes', SCHNORRSIG: 'yes', MUSIG: 'yes', ELLSWIFT: 'yes', EXPERIMENTAL: 'yes', ECDSA_S2C: 'yes', RANGEPROOF: 'yes', SURJECTIONPROOF: 'yes', WHITELIST: 'yes', GENERATOR: 'yes', ECDSAADAPTOR: 'yes', BPPP: 'yes', SCHNORRSIG_HALFAGG: 'yes', FROST: 'yes', CHILLDKG: 'yes', ICEBERG: 'yes', PREFRACTAL: 'yes', CC: 'gcc', WRAPPER_CMD: 'valgrind --error-exitcode=42', SECP256K1_TEST_ITERS: 2 }
+          - { WIDEMUL: 'int128', RECOVERY: 'yes', ECDH: 'yes', EXTRAKEYS: 'yes', SCHNORRSIG: 'yes', MUSIG: 'yes', ELLSWIFT: 'yes', EXPERIMENTAL: 'yes', ECDSA_S2C: 'yes', RANGEPROOF: 'yes', SURJECTIONPROOF: 'yes', WHITELIST: 'yes', GENERATOR: 'yes', ECDSAADAPTOR: 'yes', BPPP: 'yes', SCHNORRSIG_HALFAGG: 'yes', FROST: 'yes', CHILLDKG: 'yes', ICEBERG: 'yes', PREFRACTAL: 'yes', CPPFLAGS: '-DVERIFY', CTIMETESTS: 'no' }
           - BUILD: 'distcheck'
 
     steps:
@@ -762,6 +772,7 @@ jobs:
       FROST: 'yes'
       CHILLDKG: 'yes'
       ICEBERG: 'yes'
+      PREFRACTAL: 'yes'
 
     steps:
       - *CHECKOUT