build: apply the review-fixes build conventions to prefractal

The prefractal module landed on main in parallel with the review-fixes
branch, so neither of that branch's two build changes covered it:

- CMake gated frost, chilldkg and iceberg on SECP256K1_EXPERIMENTAL but
  not prefractal, leaving CMake accepting a configuration that
  configure.ac already rejects.
- doc/prefractal.md was referenced by no build file, so it would be
  absent from a release tarball alongside the other module docs.

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

CMakeLists.txt

@@ -139,6 +139,9 @@ if(NOT SECP256K1_EXPERIMENTAL)
   if(SECP256K1_ENABLE_MODULE_ICEBERG)
     message(FATAL_ERROR "Iceberg module is experimental. Use -DSECP256K1_EXPERIMENTAL=ON to allow.")
   endif()
+  if(SECP256K1_ENABLE_MODULE_PREFRACTAL)
+    message(FATAL_ERROR "Prefractal module is experimental. Use -DSECP256K1_EXPERIMENTAL=ON to allow.")
+  endif()
 endif()
 
 set(SECP256K1_VALGRIND "AUTO" CACHE STRING "Build with extra checks for running inside Valgrind. [default=AUTO]")

Makefile.am

@@ -318,7 +318,7 @@ maintainer-clean-local: clean-testvectors
 ### Additional files to distribute
 EXTRA_DIST = autogen.sh CHANGELOG.md SECURITY.md
 EXTRA_DIST += doc/release-process.md doc/safegcd_implementation.md
-EXTRA_DIST += doc/ellswift.md doc/musig.md doc/iceberg.md
+EXTRA_DIST += doc/ellswift.md doc/musig.md doc/iceberg.md doc/prefractal.md
 EXTRA_DIST += src/modules/frost/frost.md src/modules/chilldkg/chilldkg.md
 EXTRA_DIST += examples/EXAMPLES_COPYING
 EXTRA_DIST += sage/gen_exhaustive_groups.sage