script: add BIP360 P2MR validation, policy plumbing, and tests #35109

What

Adds draft BIP360 P2MR support plumbing across validation/policy/type handling and tests.

Scope

• witness v2 P2MR validation path in script interpreter
• policy/standardness handling for P2MR spends
• WITNESS_V2_P2MR type plumbing updates
• functional test feature_p2mr.py and related unit test updates

Test Coverage

• ctest --output-on-failure -j6
• targeted functional tests including feature_p2mr.py, feature_taproot.py, mempool package tests
• test_runner.py --extended --jobs=6
• compatibility/tool tests requiring extra build/release artifacts

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