feat: implement BIP-340 Schnorr signing for secp256k1

[jiayupu-cb]

Fixes # .

Note on file count

99 of the 106 changed files are mechanical copyright-year updates (2025 → 2026) across types/, client/, and server/. These are auto-generated files — make check-gen runs addlicense inside Docker and stamps all regenerated files with the current year, so any PR opened in 2026 would include this diff. This is a pre-existing repo-wide issue that will be addressed separately.

Files worth reviewing:

• keys/signer_secp256k1.go
• keys/signer_secp256k1_test.go
• asserter/construction.go
• asserter/construction_test.go
• go.mod / go.sum

---

Motivation

SchnorrBip340 ("schnorr_bip340") was already defined as a SignatureType
constant in the types module but was never implemented — Sign() and Verify()
both returned ErrSignUnsupportedSignatureType. This PR completes the
implementation so callers can actually use BIP-340 Schnorr signatures (used by
Bitcoin Taproot key-path spending) through the Mesh SDK.

Solution

keys/signer_secp256k1.go — Implement Sign and Verify for
types.SchnorrBip340 using github.com/btcsuite/btcd/btcec/v2/schnorr, which
is a spec-compliant BIP-340 implementation:

• Sign: validates the message is exactly 32 bytes (BIP-340 requirement), then
  calls schnorr.Sign(privKey, msg) — handles key-parity negation and tagged-hash
  nonce derivation internally. Returns 64 bytes (bytes(R.x) || bytes(s)).
• Verify: parses the 33-byte compressed public key via btcec.ParsePubKey,
  then verifies with schnorr.Signature.Verify, which lifts the key to its
  even-Y form per the BIP-340 spec.

asserter/construction.go — Add types.SchnorrBip340 to the
SignatureType() allowed-list so the asserter accepts it in payloads and
returned signatures.

go.mod — Bump github.com/coinbase/rosetta-sdk-go/types from v1.7.1
to v1.7.2, which renames the constant from SCHNORR_BIP340 to SchnorrBip340
for PascalCase consistency (see PR #546).

Tests added:

• TestSignSecp256k1: sign with SchnorrBip340, assert 64-byte output.
• TestVerifySecp256k1: BIP-340 round-trip (sign → verify).
• TestSchnorrBip340_Bip341KeyPathVector: uses a tweaked private key and sighash
  from the BIP-341 wallet test vectors (keyPathSpending[0], txinIndex=4,
  hashType=0 / SIGHASH_DEFAULT). Checks (1) that our Sign produces a valid
  64-byte signature that self-verifies, and (2) that our Verify accepts the
  spec's expected witness bytes — confirming interoperability with external
  BIP-340 implementations.
• TestSchnorrBip340_VerifyNegative: rejects garbage signature bytes, a
  signature over the wrong message, and a signature verified against the wrong
  public key.
• TestSchnorrBip340_SignNegative: rejects messages that are not exactly 32
  bytes (5, 31, 33, 64 bytes tested).
• asserter/construction_test.go: explicit asserter test case for
  types.SchnorrBip340 in TestSigningPayload.

Open questions

None.

[cb-heimdall]

✅ Heimdall Review Status

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