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

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