make TreeSitterService a shared singleton

Each ClideCodeBlock and EditorView created its own TreeSitterService,
each allocating a WASM engine, store, parser, and cursor via FFI.
When widgets were disposed, multiple instances freeing the same
underlying native resources caused double-free corruption on startup.

Single shared instance for the app lifetime.  Also fixes overlapping
tree-sitter spans that caused duplicated text in code blocks — spans
are now clipped to avoid re-emitting already-rendered characters.

Co-Authored-By: Claude <noreply@anthropic.com>

lib/builtin/editor/src/editor_view.dart

@@ -27,7 +27,7 @@ class EditorView extends StatefulWidget {
 
 class _EditorViewState extends State<EditorView> {
   EditorController? _controller;
-  final TreeSitterService _syntax = TreeSitterService();
+  final TreeSitterService _syntax = TreeSitterService.shared;
   late final SyntaxTextController _text;
   late final FocusNode _focus;
   String? _lastRemoteContent;
@@ -57,7 +57,6 @@ class _EditorViewState extends State<EditorView> {
     _focus.dispose();
     _controller?.removeListener(_onControllerChanged);
     _controller?.dispose();
-    _syntax.dispose();
     super.dispose();
   }