Full virtualization of work‑item tree

[SorraTheOrc]

Summary

Replace the current list rendering with a virtual‑scroll implementation that only materializes visible rows, improving performance for large work‑item trees.

Acceptance Criteria

• Benchmark harness shows ≤ 200 ms latency even when the JSONL file grows to ~5 k items.
• Scrolling remains smooth and UI state (expanded nodes, selections) persists correctly.

Minimal Implementation

• Introduce a small virtual‑list library (e.g., blessed-contrib or a custom viewport) into src/tui/layout.ts as an alternative renderer.
• Wire it via a --virtualize flag.
• Add unit tests for the virtual list’s indexing logic.