Add P2MR (BIP360) payment support

[BlobMaster41]

Description

Implement Pay-to-Merkle-Root (P2MR, BIP360) support: add P2MR payment class and legacy factory (src/payments/p2mr.ts), types (P2MRPayment), and tests/fixtures. Wire P2MR into address and PSBT utilities (address.fromOutputScript/toOutputScript, psbt utils, payments index) and add Merkle utilities (rootHashFromPathP2MR and tapBranchHash) reused from Taproot logic. Include documentation (documentation/p2mr.md) and update package metadata (package-lock.json) for a new release candidate.

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 implements Pay-to-Merkle-Root (P2MR, referenced as BIP 360) payment support - a SegWit version 2 output type that commits directly to the Merkle root of a script tree without an internal public key. This eliminates the quantum-vulnerable key-path spend found in P2TR (Taproot) while maintaining the efficient script tree structure.

Changes:

• Added P2MR payment class and legacy factory function with full validation and lazy-computed getters following established patterns
• Implemented P2MR-specific Merkle utilities (rootHashFromPathP2MR) and exported tapBranchHash for reuse
• Integrated P2MR into address encoding/decoding, PSBT utilities, type system, and exports with comprehensive test fixtures and documentation

Reviewed changes

Copilot reviewed 10 out of 11 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/payments/p2mr.ts	New P2MR payment class implementation with validation, getters, factory methods, and legacy function
src/payments/types.ts	Added P2MRPayment interface and P2MR to PaymentType enum and Payment union
src/payments/index.ts	Exported P2MR types, classes, functions, and utilities (rootHashFromPathP2MR, tapBranchHash)
src/payments/bip341.ts	Added rootHashFromPathP2MR function and made tapBranchHash public for P2MR support
src/address.ts	Added P2MR detection in fromOutputScript and encoding in toOutputScript for bc1z addresses
src/psbt/psbtutils.ts	Added isP2MR payment factory function for PSBT integration
src/index.ts	Exported P2MRPayment type in main index
test/payments.spec.ts	Added p2mr module to test execution array
test/fixtures/p2mr.json	Comprehensive test fixtures covering address/output/hash conversions, script trees, and witness construction
documentation/p2mr.md	Complete documentation with examples, API reference, and comparison with P2TR
package-lock.json	Version bump from 7.0.0-rc.1 to 7.0.0-rc.2

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