Add P2MR (BIP360) support to PSBT handling

[BlobMaster41]

Description

Detect and handle P2MR (Pay-to-Merkle-Root) outputs throughout PSBT logic. Highlights:

• Add isP2MRInput helper and treat P2MR as taproot-style input in isTaprootInput.
• Avoid key-path finalization for P2MR inputs in Psbt.finalizeInput (always use script-path).
• Import rootHashFromPathP2MR and isP2MR, and extend tap leaf verification to support P2MR control-block format. isTapLeafInTree now checks P2TR and P2MR control-block shapes and validates against the merkle root.
• Update checks for mixed taproot/non-taproot outputs to account for P2MR.
• Extensive tests added for P2MR: detection, single- and two-leaf signing/finalization, signature validation, and rejecting key-path finalization.

These changes enable BIP-360 P2MR script-path spends to be signed, validated, finalized, and extracted via PSBT.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Performance improvement
• Refactoring (no functional changes)
• Documentation update
• CI/CD changes
• Dependencies update

Checklist

Build & Tests

• npm install completes without errors
• npm test passes all tests
• npm run browserBuild completes without errors (if applicable)

Code Quality

• Code follows the project's coding standards
• No new compiler/linter warnings introduced
• Error handling is appropriate
• TypeScript types are properly defined

Documentation

• Code comments added for complex logic
• Public APIs are documented
• README updated (if applicable)

Security

• No sensitive data (keys, credentials) committed
• No new security vulnerabilities introduced
• Cryptographic operations reviewed (if applicable)

Bitcoin Specific

• Transaction serialization/deserialization is correct
• Script operations are properly validated
• Network parameters are handled correctly
• PSBT operations maintain compatibility

Testing

Related Issues

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[Copilot]

Pull request overview

This PR adds comprehensive support for P2MR (Pay-to-Merkle-Root, BIP360) to the PSBT handling logic. P2MR is a SegWit v2 output type that commits directly to a Merkle root without an internal public key, enabling script-path spending only (no key-path). The implementation extends the existing taproot infrastructure to handle P2MR alongside P2TR throughout the PSBT signing, validation, and finalization workflows.

Changes:

• Adds P2MR detection and handling throughout PSBT input/output validation using isP2MRInput helper
• Prevents key-path finalization for P2MR inputs (always uses script-path)
• Extends tap leaf verification with P2MR control-block format support via rootHashFromPathP2MR
• Adds comprehensive test coverage for P2MR detection, signing, finalization, and validation

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/psbt/bip371.ts	Adds isP2MRInput helper; extends isTaprootInput and isTaprootOutput to detect P2MR; updates isTapLeafInTree to verify P2MR control blocks using rootHashFromPathP2MR with fallback logic for ambiguous cases
src/psbt.ts	Modifies #finalizeTaprootInput to prevent key-path finalization for P2MR inputs by checking !isP2MRInput(input) before using tapKeySig
test/psbt.spec.ts	Adds comprehensive P2MR test suite covering detection via isTaprootInput, single/multi-leaf script-path signing and finalization, signature validation, and rejection of key-path finalization

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