Document retrieval benchmark fixes and PASS

[anusbutt]

Outcome

Updates CHANGELOG.md with the retrieval-first guidance, concrete stopping rule, and GraphKeeper Memory Bench #1 post-fix PASS showing FAIL → fix → FAIL → fix → PASS.

Evidence

• Changelog-only commit rebased onto current main.
• Benchmark PASS record is already present on main.

Checklist

• Documentation reflects the completed retrieval behavior changes and benchmark result.
• No implementation, schema, application, or benchmark files changed.
• Tests were not run for this changelog-only update.
• No unrelated tracked files are included.

Compatibility and recovery

Documentation only; no runtime or recovery behavior changes. Revert the changelog commit to roll back.