Finalize MRMA v0.4.2 correctness baseline

[0xmrma]

Summary

• bind legacy exploratory mutations to the immutable workflow baseline
• enforce cross-origin cookie policy on the final HTTPX-built request and reset denied source state
• recompute effective-plan digests and bind bundle results to RUN_PLANNED journal events
• restore the frozen benchmark v1 schema and make build_experiment_v7() emit real v7 evidence
• redesign the README and synchronize v0.4.2 documentation and release metadata

Verification

• 303 tests pass
• 70% repository branch coverage
• 95.11% critical-runtime coverage; every gated module remains above 90%
• 86.84% corrected-core coverage
• 24/24 semantic mutants killed
• 22/22 benchmark cases pass across 942 authorized attempts
• Ruff, strict mypy, corrected-core mypy, compileall, schema validation, dependency audit, wheel/sdist build, Twine, and clean-wheel resource checks pass
• Python 3.13 Linux AMD64/ARM64 container locks resolve with exact hashes

Local Docker execution was unavailable because Docker Desktop was not running; the package workflow will perform the multi-platform container build before release.