feat: virtual-scroll rendering for work-item list via --virtualize flag

[Copilot]

Blessed materializes every row as a DOM node on setItems(), making large lists (~5k items) unusably slow. This adds an opt-in virtual-scroll path that only passes the visible viewport slice to blessed.

New: VirtualList (src/tui/virtual-list.ts)

Pure indexing class with no blessed dependency — all viewport arithmetic lives here.

const vl = new VirtualList({ totalItems: 5000, viewportHeight: 20 });
vl.selectAbsolute(2500);        // jumps to global index, scrolls viewport
vl.slice(allLines);             // returns the 20-item window to pass to blessed
vl.selectedIndexInViewport;     // viewport-relative index for list.select()

Controller wiring (src/tui/controller.ts)

• Added getGlobalSelectedIndex() — converts list.selected (viewport-relative when virtual) to the full-list index. Replaces every list.selected as number call site.
• renderListAndDetail: when vl is set, updates viewport height from the widget, calls vl.slice(lines) instead of passing all lines, and syncs list.select(vl.selectedIndexInViewport).
• listKeypressHandler: detects when the cursor hits the viewport edge and calls vl.scrollBy(±1) + re-render rather than letting blessed silently ignore the keypress.
• Mouse-click handler: converts the viewport-relative click row to a global index before calling renderListAndDetail.
• updateListSelection: applies vl.offset + idx conversion before updating the detail pane.

Layout / CLI

• CreateLayoutOptions.virtualize?: boolean — when set, createLayout returns a pre-configured VirtualList in layout.virtualList.
• wl tui --virtualize flag wires this through controller.start().

Tests

27 unit tests in test/tui/virtual-list.test.ts covering moveBy, selectAbsolute, scrollBy, slice, setTotalItems, setViewportHeight, and boundary conditions. Existing TUI tests unaffected.