chilldkg: Phase 0 - module scaffolding and build wiring

Add an empty, experimental `chilldkg` module as the foundation for a
ChillDKG implementation (distributed key generation for FROST) per the
bip-frost-dkg BIP draft (v0.3.0-dev):
https://github.com/BlockstreamResearch/bip-frost-dkg

The module lives in src/modules/chilldkg/ (separate from the frost
module, per the implementation plan in .idea/docs/
chilldkg-implementation-plan.md: FROST signing (BIP 445) and ChillDKG
are separate BIPs with separate reference repos, test vectors and
review cycles; the dependency between them is one-way bytes).

New files:
- include/secp256k1_chilldkg.h: public header skeleton with the same
  "EXTREMELY DANGEROUS / work in progress" warning style as
  secp256k1_frost.h, plus a note that the BIP is a draft and tagged
  hashes/wire formats may change. No API yet (Phase 3+).
- src/modules/chilldkg/main_impl.h: implementation skeleton including
  the public header.
- src/modules/chilldkg/tests_impl.h: trivial scaffolding unit test
  (chilldkg_scaffolding_test) registered via the tests_chilldkg[]
  CASE1 array used by this repo's unit-test framework.
- src/modules/chilldkg/Makefile.am.include: autotools file list,
  mirroring the frost module's.
- src/modules/chilldkg/chilldkg.md: module doc stub (purpose, draft
  status, dependency on the schnorrsig and ecdh modules).

Build wiring (mirrors the frost module exactly):
- configure.ac: --enable-module-chilldkg (default no, experimental
  gate), dependency errors when schnorrsig or ecdh are explicitly
  disabled, AM_CONDITIONAL(ENABLE_MODULE_CHILLDKG), summary line.
- Makefile.am: include src/modules/chilldkg/Makefile.am.include under
  ENABLE_MODULE_CHILLDKG.
- src/secp256k1.c: guarded include of modules/chilldkg/main_impl.h
  after the frost module.
- src/tests.c: guarded include of tests_impl.h and
  MAKE_TEST_MODULE(chilldkg) registration.
- CMakeLists.txt: SECP256K1_ENABLE_MODULE_CHILLDKG option (OFF) +
  summary line.
- src/CMakeLists.txt: dependency checks on
  SECP256K1_ENABLE_MODULE_SCHNORRSIG and SECP256K1_ENABLE_MODULE_ECDH,
  ENABLE_MODULE_CHILLDKG=1 compile definition, public header export.

Verified:
- ./autogen.sh && ./configure --enable-experimental
  --enable-module-chilldkg --enable-module-schnorrsig
  --enable-module-ecdh && make check: PASS 3/3 (tests, noverify_tests,
  exhaustive_tests).
- configure fails with a clear error when schnorrsig or ecdh are
  disabled, or when experimental is not enabled.
- CMake build with SECP256K1_ENABLE_MODULE_CHILLDKG=ON: ctest 345/345
  passed; dependency errors fire correctly when schnorrsig/ecdh OFF.

configure.ac

@@ -245,6 +245,11 @@ AC_ARG_ENABLE(module_frost,
     [],
     [SECP_SET_DEFAULT([enable_module_frost], [no], [yes])])
 
+AC_ARG_ENABLE(module_chilldkg,
+    AS_HELP_STRING([--enable-module-chilldkg],[enable ChillDKG module (experimental)]),
+    [],
+    [SECP_SET_DEFAULT([enable_module_chilldkg], [no], [yes])])
+
 # Test-only override of the (autodetected by the C code) "widemul" setting.
 # Legal values are:
 #  * int64 (for [u]int64_t),
@@ -551,6 +556,18 @@ if test x"$enable_module_frost" = x"yes"; then
   enable_module_schnorrsig=yes
 fi
 
+if test x"$enable_module_chilldkg" = x"yes"; then
+  if test x"$enable_module_schnorrsig" = x"no"; then
+    AC_MSG_ERROR([Module dependency error: You have disabled the schnorrsig module explicitly, but it is required by the chilldkg module.])
+  fi
+  if test x"$enable_module_ecdh" = x"no"; then
+    AC_MSG_ERROR([Module dependency error: You have disabled the ecdh module explicitly, but it is required by the chilldkg module.])
+  fi
+  SECP_CONFIG_DEFINES="$SECP_CONFIG_DEFINES -DENABLE_MODULE_CHILLDKG=1"
+  enable_module_schnorrsig=yes
+  enable_module_ecdh=yes
+fi
+
 if test x"$enable_external_default_callbacks" = x"yes"; then
   SECP_CONFIG_DEFINES="$SECP_CONFIG_DEFINES -DUSE_EXTERNAL_DEFAULT_CALLBACKS=1"
 fi
@@ -598,6 +615,9 @@ if test x"$enable_experimental" = x"no"; then
   if test x"$enable_module_frost" = x"yes"; then
     AC_MSG_ERROR([FROST module is experimental. Use --enable-experimental to allow.])
   fi
+  if test x"$enable_module_chilldkg" = x"yes"; then
+    AC_MSG_ERROR([ChillDKG module is experimental. Use --enable-experimental to allow.])
+  fi
 fi
 
 # Check for concurrency support (tests only)
@@ -636,6 +656,7 @@ AM_CONDITIONAL([ENABLE_MODULE_ECDSA_ADAPTOR], [test x"$enable_module_ecdsa_adapt
 AM_CONDITIONAL([ENABLE_MODULE_BPPP], [test x"$enable_module_bppp" = x"yes"])
 AM_CONDITIONAL([ENABLE_MODULE_SCHNORRSIG_HALFAGG], [test x"$enable_module_schnorrsig_halfagg" = x"yes"])
 AM_CONDITIONAL([ENABLE_MODULE_FROST], [test x"$enable_module_frost" = x"yes"])
+AM_CONDITIONAL([ENABLE_MODULE_CHILLDKG], [test x"$enable_module_chilldkg" = x"yes"])
 AM_CONDITIONAL([USE_REDUCED_SURJECTION_PROOF_SIZE], [test x"$use_reduced_surjection_proof_size" = x"yes"])
 AM_CONDITIONAL([USE_EXTERNAL_ASM], [test x"$enable_external_asm" = x"yes"])
 AM_CONDITIONAL([USE_ASM_ARM], [test x"$set_asm" = x"arm32"])
@@ -678,6 +699,7 @@ echo "  module ecdsa-adaptor    = $enable_module_ecdsa_adaptor"
 echo "  module bppp             = $enable_module_bppp"
 echo "  module schnorrsig-halfagg = $enable_module_schnorrsig_halfagg"
 echo "  module frost            = $enable_module_frost"
+echo "  module chilldkg         = $enable_module_chilldkg"
 echo
 echo "  asm                     = $set_asm"
 echo "  ecmult window size      = $set_ecmult_window"