TIER-2 READ conductor/code_styleguides/error_handling.md end-to-end before Phase 10: refactor(gui_2): migrate L7271 render_task_dag_panel cycle_check to Result[T] (Phase 10 site 12)

Extracted _dag_cycle_check_result(app) -> Result[bool] helper above the
call site in render_task_dag_panel.
ANTI-SLIMING: full Result[T] propagation (NO except+pass). The helper
returns Result(data=has_cycle) on success (True/False) or
Result(data=False, errors=[ErrorInfo]) on exception (logging NOT a drain
per the user's principle 2026-06-17).

The legacy render_task_dag_panel code preserves its signature, calls the
helper, opens the 'Cycle Detected!' popup only when the helper returns
Result(data=True), and drains errors to app._last_request_errors.

Tests: 3 new tests verify no-cycle, cycle-detected, and RuntimeError paths.

Audit: L7271 reclassified from INTERNAL_SILENT_SWALLOW (1 site remaining,
was 2). New helper L7271 is INTERNAL_COMPLIANT.

src/gui_2.py

@@ -7267,15 +7267,38 @@ def render_task_dag_panel(app: App) -> None:  # 4. Task DAG Visualizer
    ed.end_delete()
   
   # Validate DAG after any changes
-  #TODO(Ed): Exception(Review)
-  try:
-   from src.dag_engine import TrackDAG #TODO(Ed) Reivew local import
-   ticket_dicts = [{'id': str(t.get('id', '')), 'depends_on': t.get('depends_on', [])} for t in app.active_tickets]
-   temp_dag     = TrackDAG(ticket_dicts)
-   if temp_dag.has_cycle():
-    imgui.open_popup("Cycle Detected!")
-  except Exception:
-   pass
+  cycle_result = _dag_cycle_check_result(app)
+  if not cycle_result.ok:
+   if not hasattr(app, '_last_request_errors'): app._last_request_errors = []
+   app._last_request_errors.append(("render_task_dag_panel.cycle_check", cycle_result.errors[0]))
+  elif cycle_result.data:
+   imgui.open_popup("Cycle Detected!")
+
+def _dag_cycle_check_result(app: "App") -> Result[bool]:
+ """Drain-aware variant of render_task_dag_panel DAG cycle check (L7271 INTERNAL_SILENT_SWALLOW).
+
+ Extracts the TrackDAG construction + has_cycle check try/except from
+ render_task_dag_panel into a Result-returning helper. On a valid DAG,
+ returns Result(data=False). On a cyclic DAG, returns Result(data=True).
+ On exception (bad ticket dict, dag engine failure), converts to
+ ErrorInfo (logging NOT a drain per the user's principle 2026-06-17).
+ The caller drains to app._last_request_errors.
+
+ [C: src/gui_2.py:render_task_dag_panel (L7271 caller)]
+ """
+ from src.dag_engine import TrackDAG
+ try:
+  ticket_dicts = [{'id': str(t.get('id', '')), 'depends_on': t.get('depends_on', [])} for t in app.active_tickets]
+  temp_dag     = TrackDAG(ticket_dicts)
+  has_cycle    = temp_dag.has_cycle()
+  return Result(data=has_cycle)
+ except Exception as e:
+  return Result(data=False, errors=[ErrorInfo(
+   kind=ErrorKind.INTERNAL,
+   message=f"DAG cycle check failed: {e}",
+   source="gui_2._dag_cycle_check_result",
+   original=e,
+  )])
   
   ed.end()
   # 5. Add Ticket Form