feat(gui_2): add 3 drain-plane render functions (Phase 2, tasks 2.1-2.3)

TIER-2 READ conductor/code_styleguides/error_handling.md end-to-end before Phase 2.

Adds the drain plane that consumes the 8 controller error attributes
(the data plane added by sub-track 3 Phase 6).

Module-level functions in src/gui_2.py (lines 7293-7410):
- _drain_normalize_errors (helper, lines 7295-7326): duck-typed
  normalizer for 3 error-container shapes (Optional[ErrorInfo],
  List[Tuple[str, ErrorInfo]], Dict[str, ErrorInfo])
- render_controller_error_modal (lines 7328-7368): FR-DP-1 Pattern 2
  drain point; reads all 8 controller attrs, opens per-attr popups
- _render_worker_error_indicator (lines 7370-7385): FR-DP-2 status-bar
  widget showing worker error count, clickable
- _render_last_request_errors_modal (lines 7387-7409): FR-DP-3 per-request
  error modal opened after AI request completion

App class delegation wrappers (lines 1138-1148):
- App._render_controller_error_modal -> module-level
- App._render_worker_error_indicator -> module-level
- App._render_last_request_errors_modal -> module-level

Per UI Delegation Pattern: App class has thin wrappers; logic at
module level for hot-reload support. 1-space indentation, CRLF.

Audit: no new violations introduced (gui_2.py still 25 V + 13 S +
2 RETHROW + 2 UNCLEAR + 12 COMPLIANT = 54). Tests: 4/4 pass.

tests/test_gui_2_result.py

@@ -116,4 +116,88 @@ def test_phase_1_audit_has_42_migration_target_sites():
   f"expected {EXPECTED_SITE_COUNT}. Categories seen: "
   f"{sorted({f.get('category') for f in migration_sites})}. "
   f"This must match the 42 sites declared in PHASE1_SITE_INVENTORY.md."
- )
+ )
+
+
+def test_phase_2_invariant_drain_plane_render_functions_exist():
+ """
+ Verify the 3 new module-level render functions exist in src/gui_2.py:
+ - render_controller_error_modal
+ - _render_worker_error_indicator
+ - _render_last_request_errors_modal
+
+ These are the drain-plane functions added in Phase 2 of the
+ result_migration_gui_2_20260619 track. They read the 8 controller
+ error attributes (added by sub-track 3 Phase 6) and surface them
+ to the user via ImGui popups and indicators.
+
+ The test imports src.gui_2 and inspects the module for the function
+ names. A failure here means the drain-plane wiring is incomplete.
+ """
+ import inspect
+ import src.gui_2 as gui2_mod
+ assert hasattr(gui2_mod, "render_controller_error_modal"), (
+  "src/gui_2.py is missing the module-level function "
+  "'render_controller_error_modal'. This is the FR-DP-1 drain plane "
+  "function; it must be added per the result_migration_gui_2_20260619 "
+  "Phase 2 spec."
+ )
+ assert hasattr(gui2_mod, "_render_worker_error_indicator"), (
+  "src/gui_2.py is missing the module-level function "
+  "'_render_worker_error_indicator'. This is the FR-DP-2 drain plane "
+  "function; it must be added per the result_migration_gui_2_20260619 "
+  "Phase 2 spec."
+ )
+ assert hasattr(gui2_mod, "_render_last_request_errors_modal"), (
+  "src/gui_2.py is missing the module-level function "
+  "'_render_last_request_errors_modal'. This is the FR-DP-3 drain plane "
+  "function; it must be added per the result_migration_gui_2_20260619 "
+  "Phase 2 spec."
+ )
+ assert callable(getattr(gui2_mod, "render_controller_error_modal")), (
+  "render_controller_error_modal exists but is not callable."
+ )
+ assert callable(getattr(gui2_mod, "_render_worker_error_indicator")), (
+  "_render_worker_error_indicator exists but is not callable."
+ )
+ assert callable(getattr(gui2_mod, "_render_last_request_errors_modal")), (
+  "_render_last_request_errors_modal exists but is not callable."
+ )
+
+
+def test_phase_2_invariant_drain_plane_app_delegations_exist():
+ """
+ Verify the 3 new App class delegation methods exist in src/gui_2.py:
+ - App._render_controller_error_modal
+ - App._render_worker_error_indicator
+ - App._render_last_request_errors_modal
+
+ Per conductor/product-guidelines.md §"UI Delegation for Hot-Reload",
+ the App class must contain only thin delegation wrappers; the actual
+ logic lives in module-level functions. This test locks the
+ delegation contract for Phase 2.
+
+ The test imports src.gui_2, gets the App class via the module
+ (lazily - via the _LazyModule path or directly), and checks for
+ the methods.
+ """
+ import src.gui_2 as gui2_mod
+ app_cls = getattr(gui2_mod, "App", None)
+ assert app_cls is not None, (
+  "src.gui_2 has no 'App' class attribute. Cannot verify delegations."
+ )
+ for method_name in (
+  "_render_controller_error_modal",
+  "_render_worker_error_indicator",
+  "_render_last_request_errors_modal",
+ ):
+  assert hasattr(app_cls, method_name), (
+   f"App class is missing delegation method '{method_name}'. "
+   f"The drain plane requires the App class to delegate to the "
+   f"module-level render functions so the UI delegation pattern "
+   f"supports hot-reload."
+  )
+  method = getattr(app_cls, method_name)
+  assert callable(method), (
+   f"App.{method_name} exists but is not callable."
+  )