Implement Taproot key-path wallet signing

[mswilkison]

Summary

Stacked on #3866.

This PR implements Taproot-native key-path wallet signing for the FROST migration path. It adds P2TR script handling, BIP341 SIGHASH_DEFAULT computation, BIP340 Schnorr signature verification, and single-element Taproot witness application in the Bitcoin transaction builder.

The wallet transaction executor now routes all-P2TR transactions through the Schnorr/Taproot witness path. Mixed Taproot plus legacy inputs are rejected before signing, so this does not introduce a dual-signing model.

Details

• Add P2TR script helpers and x-only output key extraction.
• Add Taproot key-path sighash generation without a repo-wide btcd upgrade.
• Add AddTaprootKeyPathSignatures for 64-byte BIP340 signatures.
• Preserve canonical 32-byte FROST signing messages when big.Int strips leading zero bytes.
• Add builder and wallet tests covering all-P2TR signing and mixed-input rejection.

Validation

• go test ./pkg/bitcoin ./pkg/tbtc
• go test -tags=frost_native ./pkg/frost/signing

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