crypto: Ethereum primitives for SimpleX names

secp256k1 (vendored libsecp256k1 v0.8.0, recoverable ECDSA), BIP-39
mnemonics, BIP-32 derivation, Keccak-256, EIP-55 addresses and EIP-712
typed data hashing.

Client-side signing only: no RLP, no transaction construction, no chain
writes. The resolver path stays read-only.

Verified against published vectors — the 24 official BIP-39 English
vectors, BIP-32 spec vectors 1 and 2, the EIP-55 spec addresses and the
EIP-712 Mail example.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.gitmodules

@@ -4,3 +4,6 @@
 [submodule "cbits/blst"]
 	path = cbits/blst
 	url = https://github.com/supranational/blst.git
+[submodule "cbits/libsecp256k1"]
+	path = cbits/libsecp256k1
+	url = https://github.com/bitcoin-core/secp256k1.git