Add line layout cache with rendering regression tests

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Summary

• Add LineLayoutCache to cache CTLine objects and ViewLineInfo per buffer row, avoiding repeated CTLineCreateWithAttributedString calls for unchanged lines
• Add RenderingStrategy enum (.legacy, .cached) with runtime toggle via renderingStrategy property
• Add cache metrics and telemetry (LineLayoutCacheMetrics) for observability
• Add comprehensive test suite for cache behavior, invalidation, and rendering correctness

Test coverage

• LineLayoutCacheTests - cache hits/misses, invalidation, generation tracking
• RenderingStrategyTests - strategy switching, visual equivalence
• TerminalUpdateRangeTests - dirty region tracking
• ViewLineInfoTests - line info construction

Expected performance characteristics

Scenario	Cache benefit
Scrolling through static content (scrollback, less, git log)	High - lines cached by absolute buffer row, scroll back = cache hit
Mostly static display (editors, shell with visible history)	Good - only changed lines invalidated
Full-screen refresh every frame (top, htop)	Minimal - all lines invalidated each cycle

The cache keys on absolute bufferRow, not screen position, so viewport scrolling without content changes yields cache hits on revisited lines.

TODO

• Cache eviction strategy (LRU or size cap) to bound memory usage for large scrollback

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Just PR’ing as a heads up. Currently testing in my visionOS remote term app. Getting improve fps and lower latency especially with tmux panes.