Add libsecp256k1 v0.7.1 as src/secp256k1 submodule and introduce
crypto_ecdsa / crypto_ecdh wrappers as drop-in replacements for the
OpenSSL ECDSA_verify / ECDSA_sign / ECDH_compute_key call sites used
by key.cpp and smessage.cpp. Wrappers preserve on-chain compatibility
(lax DER parsing, 65-byte recoverable compact sigs, SEC1 priv-key
DER round-trip, raw-X ECDH output for smsg KDF).
CMake wires the submodule and new sources into the build. Mid-refactor;
landing as a checkpoint before stacking sync-pipeline work on top.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
