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/kernel/src/syntax/tree_sitter_service.dart

@@ -42,6 +42,9 @@ class _LoadedGrammar {
 }
 
 class TreeSitterService {
+  static final TreeSitterService shared = TreeSitterService._();
+  TreeSitterService._();
+
   final Map<String, _LoadedGrammar> _grammars = {};
   final Set<String> _unavailable = {};