Prototype incremental rendering

[SorraTheOrc]

Summary

Refactor the TUI layout to perform incremental rendering, updating only the newly visible nodes on expand/collapse instead of recomputing the entire tree.

Acceptance Criteria

• Measured latency on the benchmark harness drops below 200 ms for the 30‑item tree.
• No regression in existing UI functionality (manual smoke test passes).

Minimal Implementation

• Refactor src/tui/layout.ts to separate “full tree layout” from “visible node layout”.
• Update expandInProgressAncestors to call the new incremental layout only for affected branches.
• Disable full recompute by default, gated behind a config flag enableFullLayout.