Refactor split diff rendering around an explicit review row plan

[benvinegar]

Summary

• add buildReviewRenderPlan(...) as an explicit presentational planning layer on top of semantic Pierre diff rows
• move inline-note anchoring, guide-range membership, guide-cap placement, and hunk anchor assignment out of PierreDiffView's ad hoc render loop
• add planner-focused coverage for split note ranges, hidden hunk headers, and range-less note fallback behavior

Testing

• bun test
• bun run typecheck
• bunx oxlint . --deny-warnings
• bunx oxfmt --check .
• bun run test:tty-smoke