fix: preserve staged rename identity with unstaged edits

[rogerchappel]

Summary

• add an end-to-end regression for a staged rename followed by an unstaged edit
• preserve rename metadata while merging staged and unstaged records
• report unique logical files in the combined diff stat and document the behavior

Commits

• 338d0e3 test: cover staged rename with unstaged edits
• 3a8510e fix: retain rename identity across diff sources
• b56d259 docs: explain staged and unstaged rename plans

Verification

• npm run release:check (18 tests, smoke, package dry-run)
• git diff --check origin/main...HEAD
• commit author/committer identity verified as Roger Chappel miscanalysis@gmail.com for every proposed commit