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/gutter_renderer.py

@@ -0,0 +1,93 @@
+# Python imports
+
+# Lib imports
+import gi
+gi.require_version("Gtk", "3.0")
+gi.require_version("GtkSource", "4")
+
+from gi.repository import Gtk
+from gi.repository import GtkSource
+
+# Application imports
+from .folding_actions import collapse_range, expand_range
+
+
+
+def handle_collapse(view, fold):
+    collapse_range(view, fold)
+
+    for inner in view.fold_starts:
+        if fold["start_line"] < inner["start_line"] <= fold["end_line"]:
+            view.fold_states[inner["id"]] = False
+
+def handle_expand(view, fold):
+    expand_range(view, fold)
+
+def handle_block_toggle(collapsed, view, fold):
+    if not collapsed:
+        handle_collapse(view, fold)
+    else:
+        handle_expand(view, fold)
+
+
+def on_query_data(renderer, start_iter, end_iter, state, view):
+    line = start_iter.get_line()
+
+    if not line in view.fold_start_set:
+        renderer.set_text("", -1)
+        return
+
+    fold      = next((f for f in view.fold_starts if f["start_line"] == line), None)
+    collapsed = fold and view.fold_states.get(fold["id"], False)
+
+    renderer.set_text( "▶" if collapsed else "▼", -1 )
+
+def on_click(view, event, renderer):
+    if not event.button == 1:      return False
+    window = view.get_window(Gtk.TextWindowType.LEFT)
+    if not event.window == window: return False
+
+    x, y = view.window_to_buffer_coords(
+        Gtk.TextWindowType.LEFT, int(event.x), int(event.y)
+    )
+
+    _, iter_ = view.get_iter_at_location(x, y)
+    line     = iter_.get_line()
+
+    if line not in view.fold_start_set: return False
+
+    for fold in view.fold_starts:
+        if not fold["start_line"] == line: continue
+
+        collapsed = view.fold_states.get(fold["id"], False)
+        view.fold_states[fold["id"]] = not collapsed
+
+        handle_block_toggle(collapsed, view, fold)
+
+        renderer.queue_draw()
+        return True
+
+    return False
+
+
+def setup_gutter(view):
+    gutter = view.get_gutter(Gtk.TextWindowType.LEFT)
+    buffer = view.get_buffer()
+
+    if not buffer.get_tag_table().lookup("invisible"):
+        tag = buffer.create_tag("invisible")
+        tag.set_property("invisible", True)
+
+    view.fold_starts    = []
+    view.fold_start_set = set()
+    view.fold_states    = {}
+
+    renderer = GtkSource.GutterRendererText()
+    renderer.set_size(12)
+    renderer.set_padding(2, -1)
+
+    renderer.query_data_id = renderer.connect("query-data", on_query_data, view)
+    view.collapse_click_id = view.connect("button-press-event", on_click, renderer)
+
+    gutter.insert(renderer, 0)
+    view.fold_renderer = renderer