docs(release): add v0.5 readiness checklist

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Summary

• add a v0.5 release readiness checklist that maps each required release item to reviewer-facing evidence
• add a parser-contract anchor back to the v0.5 Evidence Explainability release path
• link the readiness checklist from the reviewer path

Boundaries

• documentation-only refinement
• no tag or GitHub Release created
• no parser, detector, CLI, or report-generation behavior changes

Validation

• cmake -S . -B build
• cmake --build build
• ctest --test-dir build -C Debug --output-on-failure
• git diff --check
• local Markdown link check for touched docs
• release readiness checklist row check
• privacy/secret scan for touched docs