Tighten the review render-plan contract

[benvinegar]

Summary

• make the single visible inline-note policy explicit in buildReviewRenderPlan()
• clarify anchor and guide selection for range-backed vs range-less notes
• expand render-plan contract coverage for deletion-only notes, multi-hunk diffs, hidden-header anchors, and multi-note metadata

Why

The render-plan refactor introduced a good abstraction, but some key behaviors were still implicit:

• only the first visible note rendered inline, but that policy lived in a bare visibleAgentNotes[0]
• range-less notes anchored correctly in practice, but the guide behavior was not clearly expressed
• tests covered the happy path more than the boundary conditions this abstraction is supposed to own

This follow-up turns those behaviors into an explicit contract so future note UX and windowing work can build on a clearer foundation.

Testing

• bun run typecheck
• bun test
• pseudo-TTY smoke run of bun run src/main.tsx diff ...