feat: searchable mode — type to filter items inline

[gfargo]

Summary

For lists with many items, typing characters to filter them inline is a major usability improvement. Similar to react-select's search behavior, but adapted for terminal rendering.

Proposed API

<EnhancedSelectInput
  items={items}
  searchable
  searchPlaceholder="Type to filter..."
/>

Behavior

• When searchable is true, printable characters are captured as a search query
• The items list is filtered to entries whose label matches the query (fuzzy or prefix)
• A search input line is rendered above the item list
• Backspace removes the last search character
• Escape clears the search
• Navigation (arrows/vim) works on the filtered subset

Challenges

• Conflict with existing hotkey system (needs careful key routing)
• Conflict with vim navigation keys (h, j, k, l)
• Filtering changes item indices, which affects selectedIndex state

Scope

• Add searchable?: boolean and searchPlaceholder?: string props
• Add query state and filter logic
• Add search input rendering
• Add tests for filtering, navigation within filtered results, and clearing
• Document in README

[gfargo]

This feature is implemented in PR #26 and ready to close.

Implementation:

• searchable?: boolean and searchPlaceholder?: string props
• Case-insensitive substring matching on item labels
• Search input renders above item list (/ query or / placeholder)
• "No matches" shown when query matches nothing
• Backspace removes last char; Escape clears query first, then calls onCancel
• Vim keys (h/j/k/l) and hotkeys disabled in searchable mode — characters go to the query
• Space treated as search character (not multi-select toggle)
• Selection resets to first match when query changes
• searchQuery exposed in UseEnhancedSelectInputResult
• Works with limit/pagination, groups, and disabled items

Tests: 22 new tests covering filtering, navigation within filtered results, clearing, edge cases, and integration with other features.

Docs: New "Searchable Mode" section in README with examples, plus props table updated.