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.

include/secp256k1_chilldkg.h

@@ -0,0 +1,38 @@
+#ifndef SECP256K1_CHILLDKG_H
+#define SECP256K1_CHILLDKG_H
+
+#include "secp256k1.h"
+#include "secp256k1_extrakeys.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#include <stddef.h>
+#include <stdint.h>
+
+/** This module implements ChillDKG, a distributed key generation (DKG)
+ *  protocol for FROST, as specified by the bip-frost-dkg BIP draft
+ *  (https://github.com/BlockstreamResearch/bip-frost-dkg, version 0.3.0-dev).
+ *
+ *  This code is currently a work in progress. It's not secure nor stable.
+ *  IT IS EXTREMELY DANGEROUS AND RECKLESS TO USE THIS MODULE IN PRODUCTION!
+ *
+ *  Moreover, the bip-frost-dkg BIP is still a draft: tagged hashes, wire
+ *  formats, and protocol details may change in future BIP versions. There is
+ *  no guarantee that this implementation will remain compatible with the
+ *  final specification.
+ *
+ *  The output of a ChillDKG session (a secret share, the threshold public
+ *  key, and the public shares of all participants) is designed to be used
+ *  directly with the FROST signing module (see include/secp256k1_frost.h).
+ *
+ *  It is recommended to read the documentation in this include file carefully.
+ *  Further notes on API usage can be found in src/modules/chilldkg/chilldkg.md.
+ */
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif