Theme previews re-render the whole transcript, and streaming re-renders
every frame, so per-line and per-session work dominated in long sessions.
- visibleWidth: fast path for ANSI-styled ASCII, allocation-free escape scanning
- Box: compare unpadded child lines and skip a second width measurement
- Markdown: reuse parsed tokens across invalidation
- footer: cache usage totals and context usage per session state
- bash results: cache the collapsed preview including the expand hint
- sanitizeBinaryOutput: single regex replace
