Implement TextASTSync and TextEditor — the core text editing layer
that bridges raw text and the structured AST. TextASTSync provides
bidirectional text-AST sync with debounce via TreeSitterParser and
generators. TextEditor wraps a text buffer with edit operations,
undo/redo that tracks AST state, find/replace, and selection.
New files: TextASTSync.h, TextEditor.h, step50_test.cpp
Updated: step51/53 tests (replaced stubs with real includes),
CMakeLists.txt (tree-sitter linking), PROGRESS.md
Fixed: step53 find-position off-by-one, canRedo assert logic
19/19 tests pass (step50: 8, step51: 5, step53: 6)
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Tests text undo/redo with bidirectional AST sync, find/replaceAll,
selection, and verifies AST stays consistent through edit operations.
Defines TextEditor interface.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
