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/folding_engine.py

@@ -0,0 +1,31 @@
+# Python imports
+
+# Lib imports
+
+# Application imports
+from .fold_types import FOLD_NODES
+
+
+
+def get_folding_ranges(lang_name: str, ast):
+    root       = ast.root_node
+    fold_types = FOLD_NODES.get(lang_name, set())
+    ranges     = []
+
+    def visit(node):
+        if node.type in fold_types:
+            start_line = node.start_point[0]
+            end_line   = node.end_point[0]
+
+            if end_line > start_line:
+                ranges.append({
+                    "start_line": start_line,
+                    "end_line": end_line,
+                    "id": (start_line, end_line, node.type),
+                })
+
+        for child in node.children:
+            visit(child)
+
+    visit(root)
+    return ranges