Cache per-file TUI row models for source and diff views

[jmqd]

Rendering block views can rebuild whole-file row content per block. Audit refs: trueflow/src/commands/tui.rs:5892, trueflow/src/commands/tui.rs:5940, trueflow/src/commands/tui.rs:6401, trueflow/src/commands/tui.rs:4732. Trigger: navigating many blocks in a large file. Current cache is per node/focus/width, so work can scale with blocks_in_file * file_lines. Direction: cache per-file source/diff row models and derive block or viewport slices without building offscreen Line values.