script: Add BIP360 P2MR validation, policy plumbing, and tests #35107

What

This PR adds draft BIP360 P2MR support wiring across script validation, policy, type solving/signing, RPC decoding, wallet integration, and test coverage.

Main changes:

• script interpreter:
  • add witness v2 P2MR validation path
  • verify P2MR commitment using P2MR control-block semantics
• standardness/policy:
  • add P2MR-related standardness checks in mempool policy paths
• script type plumbing:
  • add TxoutType::WITNESS_V2_P2MR
  • update solver/sign/address/rpc/wallet paths for the new type
• tests:
  • add functional test test/functional/feature_p2mr.py
  • register it in test/functional/test_runner.py
  • update related unit tests

Why

BIP360 P2MR introduces a witness v2 pattern that needs coherent handling across interpreter, policy, and supporting infrastructure. This change set wires those components together and adds coverage for acceptance/rejection behavior.

Scope / Non-goals

• This PR does not claim broad "quantum-safe" guarantees.
• It focuses on implementing and testing P2MR handling in Core’s current validation/policy/test framework.

Test coverage

Executed locally:

• ctest --output-on-failure -j6
• build/test/functional/test_runner.py feature_p2mr.py feature_taproot.py mempool_accept.py mempool_packages.py mempool_package_limits.py
• .venv-functional/bin/python build/test/functional/test_runner.py --extended --jobs=6
• .venv-functional/bin/python build/test/functional/test_runner.py tool_bench_sanity_check.py tool_bitcoin_chainstate.py
• .venv-functional/bin/python build/test/functional/test_runner.py wallet_backwards_compatibility.py wallet_migration.py mempool_compatibility.py feature_coinstatsindex_compatibility.py feature_unsupported_utxo_db.py

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LLM Linter (✨ experimental)

Possible places where named args for integral literals may be used (e.g. func(x, /*named_arg=*/0) in C++, and func(x, named_arg=0) in Python):

• AddCoins(coins, CTransaction{tx_create}, 0, false) in src/test/transaction_tests.cpp

^{2026-04-18 14:33:57}