feat(tree-sitter, views): initialize AST on focus and emit source view creation event

- Add set_ast helper to centralize Tree-sitter parsing logic
- Parse and attach AST on FocusedViewEvent and TextChangedEvent
- Request file from buffer on view focus before parsing
- Fix parser guard condition in get_parser (handle missing language properly)

- Emit CreatedSourceViewEvent when a new source view is added
- Register CreatedSourceViewEvent in DTO exports

- Update TODO:
  - Remove completed collapsible code blocks task
  - Add fix note for code block icon desync issue

chore:
- Add scaffolding for code_fold UI plugin
- Add created_source_view_event DTO

plugins/code/ui/code_fold/manifest.json

@@ -0,0 +1,7 @@
+{
+    "name": "Code Fold",
+    "author": "ITDominator",
+    "version": "0.0.1",
+    "support": "",
+    "requests": {}
+}