docs(release): add v0.5 evidence explainability notes

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Summary

• add docs/release-v0.5.0.md for the Evidence Explainability Release
• move current explainability / parser observability changelog entries into v0.5.0
• mark finding explainability fields as stable loglens.report.v2 JSON contract surface
• add the v0.5 release route to reviewer-facing docs and README

Boundaries

• documentation-only change
• no official tag or GitHub Release created
• no parser or detector 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-note required term check
• privacy/secret scan for touched docs