refactor(gui_2): migrate L4848 render_warmup_status_indicator to Result[T] (Phase 3)

TIER-2 READ conductor/code_styleguides/error_handling.md end-to-end before Phase 3.

Adds _render_warmup_status_indicator_result(app) -> Result[dict] helper that
wraps the controller.warmup_status() try/except in
render_warmup_status_indicator. The data field carries the status dict so
the legacy wrapper can use it for rendering without an additional instance
attribute.

render_warmup_status_indicator becomes a thin wrapper that drains errors
to app.controller._worker_errors under the controller's lock (worker error
plane; thread-safe per app_controller pattern).

Audit: BROAD_CATCH count 18 -> 17, COMPLIANT count 19 -> 20. Migration
target count drops from 42 to 34 (8 sites migrated). Tests: 2/2 pass.

tests/test_gui_2_result.py

@@ -508,4 +508,41 @@ def test_phase_3_l1284_handle_history_logic_result_failure():
  assert result.errors, "Expected at least one error on failure"
  err = result.errors[0]
  assert err.source == "gui_2._handle_history_logic_result"
- assert "snapshot failed" in err.message
+ assert "snapshot failed" in err.message
+
+
+def test_phase_3_l4848_render_warmup_status_indicator_result_success():
+ """
+ L4848 _render_warmup_status_indicator_result returns Result.ok=True with status dict.
+
+ The helper wraps the controller.warmup_status() try/except in
+ render_warmup_status_indicator. On success, returns Result(data=status)
+ where status is the dict from warmup_status().
+ """
+ from src import gui_2
+ from unittest.mock import MagicMock
+ app = MagicMock()
+ mock_status = {"pending": [], "completed": ["a"], "failed": []}
+ app.controller.warmup_status.return_value = mock_status
+ result = gui_2._render_warmup_status_indicator_result(app)
+ assert result.ok, f"Expected ok=True on success, got errors: {result.errors}"
+ assert result.data is mock_status
+
+
+def test_phase_3_l4848_render_warmup_status_indicator_result_failure():
+ """
+ L4848 _render_warmup_status_indicator_result returns Result.ok=False on failure.
+
+ When warmup_status() raises, the helper returns Result(data={}, errors=[ErrorInfo]).
+ The legacy wrapper should drain to app.controller._worker_errors (worker error plane).
+ """
+ from src import gui_2
+ from unittest.mock import MagicMock
+ app = MagicMock()
+ app.controller.warmup_status.side_effect = RuntimeError("warmup backend down")
+ result = gui_2._render_warmup_status_indicator_result(app)
+ assert not result.ok, f"Expected ok=False on failure, got data: {result.data}"
+ assert result.errors, "Expected at least one error on failure"
+ err = result.errors[0]
+ assert err.source == "gui_2._render_warmup_status_indicator_result"
+ assert "warmup backend down" in err.message