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/event-watchers/tree_sitter/tree_sitter.py

@@ -48,7 +48,7 @@ def get_parser(lang_name: str) -> Parser | None:
 
     language = LANGUAGES[lang_name]
 
-    if not language in LANGUAGES: return
+    if not language: return
 
     parser          = Parser()
     parser.language = language