perf(tui): Eliminate O(n) scans from render hot path for smooth animations

[paulbettner]

Summary

This PR eliminates O(n) complexity scans from the TUI render hot path to enable smooth 60 FPS animations and text selection during streaming sessions with 100+ messages.

Optimizations

1. Local shimmer detection - Replaced O(all-parts) scan with O(parts-in-message) by searching backwards from last message
2. Header caching - Cache token/cost calculations with dirty flags
3. Virtual viewport rendering - Viewport fetches only visible lines via callback, enabling O(1) scrolling regardless of message count
4. Fast selection updates - Selection highlighting reuses cached blocks without re-rendering markdown/syntax (10-50x speedup, 2-5ms vs 50-100ms)

Performance Impact

• Before: Laggy animations, sluggish selection (~3-10 FPS with 100+ messages)
• After: Smooth 60 FPS animations and selection updates

All optimizations include comprehensive inline documentation explaining the approach and performance characteristics.

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