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.

src/modules/chilldkg/Makefile.am.include

@@ -0,0 +1,3 @@
+include_HEADERS += include/secp256k1_chilldkg.h
+noinst_HEADERS += src/modules/chilldkg/main_impl.h
+noinst_HEADERS += src/modules/chilldkg/tests_impl.h

src/modules/chilldkg/chilldkg.md

@@ -0,0 +1,24 @@
+Notes on the chilldkg module API
+================================
+
+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). In
+a ChillDKG session, n participants jointly generate a threshold key with the
+help of an untrusted coordinator, such that no party (including the
+coordinator) learns more than its own secret share, and every participant
+obtains the threshold public key and the public shares of all participants.
+The output is designed to feed directly into the frost module
+(`include/secp256k1_frost.h`), which implements FROST signing (BIP 445) and
+explicitly leaves DKG out of scope.
+
+**This module is experimental.** Do not use it in production. The API should
+not be considered stable. The underlying BIP is still a draft (v0.3.0-dev):
+tagged hashes, wire formats, and protocol details may change in future BIP
+versions.
+
+The module depends on the schnorrsig module (for the CertEq certificate and
+proofs of possession) and the ecdh module (for encrypted share distribution).
+
+The API is under construction; see the implementation plan in
+`.idea/docs/chilldkg-implementation-plan.md` for the phased roadmap.

src/modules/chilldkg/main_impl.h

@@ -0,0 +1,11 @@
+/***********************************************************************
+ * Distributed under the MIT software license, see the accompanying    *
+ * file COPYING or https://www.opensource.org/licenses/mit-license.php.*
+ ***********************************************************************/
+
+#ifndef SECP256K1_MODULE_CHILLDKG_MAIN_H
+#define SECP256K1_MODULE_CHILLDKG_MAIN_H
+
+#include "../../../include/secp256k1_chilldkg.h"
+
+#endif

src/modules/chilldkg/tests_impl.h

@@ -0,0 +1,21 @@
+/***********************************************************************
+ * Distributed under the MIT software license, see the accompanying    *
+ * file COPYING or https://www.opensource.org/licenses/mit-license.php.*
+ ***********************************************************************/
+
+#ifndef SECP256K1_MODULE_CHILLDKG_TESTS_IMPL_H
+#define SECP256K1_MODULE_CHILLDKG_TESTS_IMPL_H
+
+#include "../../../include/secp256k1_chilldkg.h"
+
+/* Placeholder test for the chilldkg module scaffolding. Real tests are added
+ * in later implementation phases. */
+static void chilldkg_scaffolding_test(void) {
+    CHECK(1);
+}
+
+static const struct tf_test_entry tests_chilldkg[] = {
+    CASE1(chilldkg_scaffolding_test),
+};
+
+#endif