feat(tui): incremental rendering for expand/collapse and scroll

[Copilot]

The TUI rebuilds the entire visible-node tree on every expand/collapse and scroll event, causing measurable lag and occasional full event-loop lockups.

Core: visible-node cache (src/tui/state.ts)

• Added cachedVisibleNodes: VisibleNode[] | null to TuiState
• rebuildTreeState() invalidates the cache; buildVisibleNodes() populates it after a full traversal
• incrementalExpand(state, nodeIdx) — splices only the newly-visible subtree into the cache (O(subtree) instead of O(total))
• incrementalCollapse(state, nodeIdx) — removes the descendant block from the cache by index scan, no traversal needed
• Both functions fall back to a full buildVisibleNodes() when the cache is stale

// expand: only the child subtree is computed and spliced in
const newVisible = cached.slice(0, nodeIdx + 1).concat(subtree, cached.slice(nodeIdx + 1));
state.cachedVisibleNodes = newVisible;

Controller wiring (src/tui/controller.ts)

• buildVisible() changed to state.cachedVisibleNodes ?? buildVisibleNodes(state) — scroll and navigation are now O(1) lookups
• KEY_TOGGLE_EXPAND, KEY_NAV_RIGHT, KEY_NAV_LEFT handlers use the incremental functions instead of mutating state.expanded directly and triggering a full rebuild
• updateListSelection (scroll/navigate handler) now emits scroll perf events into perfMetrics when --perf is enabled

Tests (tests/tui/incremental-rendering.test.ts)

• Unit tests for incrementalExpand / incrementalCollapse covering: no-children guard, already-expanded guard, stale-cache fallback, multi-level nesting, depth-based descendant removal
• Cache invalidation regression tests
• 30-item benchmark (10 roots × 2 children): asserts median expand/collapse latency < 200 ms
• Smoke test verifying visible-node count consistency through full expand/collapse round-trips

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