chilldkg: CI wiring, ctime_tests coverage, declassify fixes

CI:
- ci/ci.sh: new CHILLDKG environment variable, passed to configure as
  --enable-module-chilldkg (mirroring FROST).
- .github/workflows/ci.yml: default CHILLDKG: 'no' and CHILLDKG: 'yes'
  in every job that enables FROST, except the x86_64 matrix entry that
  deliberately builds without the ecdh module (chilldkg requires
  schnorrsig + ecdh; the configure-time dependency error would fire
  there). YAML validity and per-job dependency presence checked
  programmatically.

ctime_tests:
- src/ctime_tests.c: run a full ChillDKG session (n = 2, t = 2) through
  the public API under the memory checker: hostpubkey_gen, params_hash,
  participant_step1, coordinator_step1, participant_step2,
  coordinator_finalize, participant_finalize, participant_recover and
  recovery_ack_sign. Host secret keys, session randomness, aux
  randomness and the resulting secret shares are undefined (secret);
  all protocol messages, the certificate, threshold public key, public
  shares, recovery data, ack signature and the secret-free state1
  objects are defined (public). state2 stays secret (contains the
  secret share).

Constant-time fixes found by running the new block under
MemorySanitizer (valgrind unavailable locally; MSan build via clang +
CMake). All are missing declassifications of secret-derived but public
(or public-outcome) values, following the frost module's
secp256k1_declassify pattern with justification comments; no real
constant-time bugs were found:
- hostpubkey_gen: declassify the computed host public key before
  serialization (public output).
- participant_step1: declassify the zero-randomness check result (only
  reveals "the RNG returned 32 zero bytes", which aborts the session).
- encpedpop participant_step1: declassify the pubnonce point before
  serialization (public, part of pmsg1).
- chilldkg_schnorrsig_sign: declassify the signer public key before
  normalization/parity branch, and declassify the return value (a
  failure only reveals a zero derived nonce, negligible probability).
- vss_commit: declassify the VSS commitments before serialization
  (public, part of pmsg1).
- vss_verify_secshare: declassify secshare*G before the infinity/eq
  checks (equals the public pubshare in honest runs; the discrete log
  is not revealed).
- simplpedpop_participant_investigate (proactive audit; not reached by
  ctime_tests): declassify the secshare-sum comparison result (the
  public fault code reveals it anyway).

Verified: MSan ctime_tests exits 0; autotools make check 10/10 (the
local tree is configured without --enable-ctime-tests because neither
valgrind nor an MSan-instrumented gcc build is available; CI runs
ctime_tests under valgrind as before); CMake ctest 428/428;
./tests --target=chilldkg and ./chilldkg_example pass.

src/modules/chilldkg/util_impl.h

@@ -203,6 +203,7 @@ static int secp256k1_chilldkg_schnorrsig_sign(const secp256k1_context *ctx, unsi
     secp256k1_ecmult_gen_ge(&ctx->ecmult_gen_ctx, &pk, &sk);
     /* The public key is not secret, so variable-time normalization and
      * branching on its y parity are fine. */
+    secp256k1_declassify(ctx, &pk, sizeof(pk));
     secp256k1_fe_normalize_var(&pk.x);
     secp256k1_fe_normalize_var(&pk.y);
     if (secp256k1_fe_is_odd(&pk.y)) {
@@ -242,6 +243,10 @@ static int secp256k1_chilldkg_schnorrsig_sign(const secp256k1_context *ctx, unsi
     secp256k1_memclear_explicit(seckey, sizeof(seckey));
     secp256k1_memclear_explicit(nonce32, sizeof(nonce32));
 
+    /* Branching on the return value only leaks whether the derived nonce is
+     * zero, which happens with negligible probability (the case of an invalid
+     * secret key is declassified above). */
+    secp256k1_declassify(ctx, &ret, sizeof(ret));
     return ret;
 }