Cache file reads for diff rendering

[krissetto]

Also adds dedicated view for collapsed reasoning blocks (so we only show + - lines on a single line instead of the full diff), and the interface for collapsed reasoning block (or just compact) renders in general so we can differentiate how other tools render when in collapsed reasoning blocks in the future

Screencast (relevant bits at ~1:00):

Screencast.From.2026-01-25.12-54-16.mp4

[krissetto]

/review

[github-actions]

Code Review Summary

Found 3 issues in the caching implementation that should be addressed:

1. Memory Leak (HIGH): Unbounded cache growth - the global cache map is never cleared
2. Race Condition (MEDIUM): Duplicate expensive work in renderEditFile on concurrent cache misses
3. Race Condition (MEDIUM): Duplicate expensive work in countDiffLines on concurrent cache misses

The collapsed view interface and nil-safety look good. The main concerns are around the cache implementation needing eviction/cleanup and proper concurrency control.

mostly irrelevant in the context of the TUI

[krissetto]

/review

[github-actions]

Code Review Summary

Reviewed the caching implementation for diff rendering. Found one potential memory management concern worth noting.

Finding

• Unbounded Cache Growth (Medium severity, line 46 in render.go)

The cache improves performance as intended, but lacks a cleanup mechanism. While the impact is modest for typical sessions, consider adding cache eviction for robustness in long-running sessions.