Foundations: Thaana (JTK) keyboard engine & useThaanaInput #8

REQUIREMENTS §13.

• Framework-agnostic engine at @javaabu/onu/thaana: phonetic (JTK-compatible) and typewriter keymaps, custom keymaps
• Pure conversion functions (keystroke + whole string)
• useThaanaInput(ref, options) for input / textarea / contenteditable; <ThaanaInput>, <ThaanaTextarea>
• Caret/selection handling, undo where possible, controlled/uncontrolled + react-hook-form
• Modifier shortcuts pass through; beforeinput-based handling for Android (keyCode 229); IME composition passthrough
• Paste: keep (default) or transliterate
• Optional Thaana/Latin toggle with screen-reader announcement
• 100% test coverage of keymaps and conversion