feat: item group support with section headers

[gfargo]

Summary

Implements #13 — adds support for visually grouping items under section headers.

Also significantly expands test coverage (75 → 106 tests).

Changes

Item Groups (#13)

• Added group?: string field to Item<V> type
• Items sharing the same group value are rendered under a shared header row
• Headers are purely visual and non-navigable (not items in the array)
• Added DefaultGroupHeaderComponent (renders ── GroupName ── with dim styling)
• Added groupHeaderComponent prop for custom header rendering
• Exported DefaultGroupHeaderComponent and GroupHeaderProperties from package entry
• Documented in README with usage examples, props table, and Item Shape

Test Coverage

• 31 new tests covering:
  • All group rendering scenarios (horizontal, multi-select, disabled items, pagination, scroll indicators)
  • Edge cases: single item, limit=1, limit > items, empty items keyboard input
  • Multi-select: Home/End, navigation, toggle workflows
  • Horizontal orientation: Home/End, scroll indicators
  • Per-item indicator visibility toggling
  • Complex object value types with explicit keys
  • Items update scenarios (grow, shrink, replace)
  • onHighlight fires on mount
  • Callbacks not provided don't crash
• Increased test delay default from 50ms → 100ms for stability under concurrent load

Testing

yarn test  # 106 tests passed

Closes #13