feat(host-agent): persist UI-tree evidence and add overlay/action visualization

Fixes issue 3: the host-agent console showed OCR results but never real
UI-tree data, because _ui_tree_nodes() checked for a get_ui_tree/ui_tree
tool action that has never existed anywhere in the codebase.

- storage/timeline.py: add a ui_tree_results field to TimelineRecord and
  Timeline.append(), mirroring the existing ocr_results field.
- runtime/task.py: _append_timeline() now extracts scene.elements with
  source == "ui" into ui_tree_results (scene_builder.build_scene() already
  preserved these; they were just never persisted).
- host_agent/web/app.py: _ui_tree_nodes() reads the new field directly
  instead of the dead tool-action check. New _overlay_payload() exposes
  each step's scene dimensions and fused element list for client-side
  rendering.
- task_detail.html: adds a toggle to overlay OCR (orange) and UI-tree
  (blue) bounding boxes on the before-action screenshot, plus a visual
  marker for the actually executed action (tap circle, or an animated
  swipe path) using an SVG viewBox so no manual coordinate-scaling JS is
  needed. Legacy/incomplete records degrade to no overlay, never an error.

Also corrects openspec/specs/runtime-task-evidence and
host-agent-console-task-pages, which had encoded the same nonexistent-tool
assumption, via the new host-agent-console-visual-evidence change.

600 tests passing; ruff/compileall/openspec validate all clean.

tests/test_host_agent_console_tasks.py

@@ -223,18 +223,8 @@ def test_authenticated_task_detail_renders_complete_step_evidence(tmp_path) -> N
         task_id=task.id,
         scene={"screen": {"width": 10, "height": 20}, "elements": []},
         prompt="inspect the current UI tree",
-        tool_call={"action": "get_ui_tree", "description": "inspect UI tree"},
-        result={
-            "result": [
-                {
-                    "id": "search",
-                    "type": "button",
-                    "text": "Search",
-                    "bounds": {"x": 1, "y": 2, "width": 3, "height": 4},
-                    "confidence": 0.98,
-                }
-            ]
-        },
+        tool_call={"action": "tap", "description": "tap search"},
+        result={"ok": True},
         before_screenshot=PNG_10X20,
         after_screenshot=PNG_10X20 + b"after",
         ocr_results=[
@@ -244,6 +234,15 @@ def test_authenticated_task_detail_renders_complete_step_evidence(tmp_path) -> N
                 "confidence": 0.95,
             }
         ],
+        ui_tree_results=[
+            {
+                "id": "search",
+                "type": "button",
+                "text": "Search",
+                "bounds": {"x": 1, "y": 2, "width": 3, "height": 4},
+                "confidence": 0.98,
+            }
+        ],
     )
 
     client, _ = _build_client(
@@ -278,3 +277,51 @@ def test_tasks_list_shows_empty_state(tmp_path) -> None:
     response = client.get("/tasks")
     assert response.status_code == 200
     assert "No executions recorded yet" in response.text
+
+
+def test_ui_tree_nodes_reads_persisted_field_directly() -> None:
+    from host_agent.web.app import _ui_tree_nodes
+
+    record = {
+        "ui_tree_results": [
+            {"id": "search", "type": "button", "text": "Search"},
+            "not-a-dict",
+        ]
+    }
+    assert _ui_tree_nodes(record) == [
+        {"id": "search", "type": "button", "text": "Search"}
+    ]
+
+
+def test_ui_tree_nodes_defaults_to_empty_for_legacy_record() -> None:
+    from host_agent.web.app import _ui_tree_nodes
+
+    assert _ui_tree_nodes({}) == []
+    assert _ui_tree_nodes({"ui_tree_results": "not-a-list"}) == []
+
+
+def test_overlay_payload_combines_scene_dimensions_and_elements() -> None:
+    from host_agent.web.app import _overlay_payload
+
+    record = {
+        "scene": {
+            "screen": {"width": 100, "height": 200},
+            "elements": [{"id": "ui-000", "source": "ui"}, "not-a-dict"],
+        }
+    }
+    assert _overlay_payload(record) == {
+        "width": 100,
+        "height": 200,
+        "elements": [{"id": "ui-000", "source": "ui"}],
+    }
+
+
+def test_overlay_payload_defaults_gracefully_for_legacy_record() -> None:
+    from host_agent.web.app import _overlay_payload
+
+    assert _overlay_payload({}) == {"width": 0, "height": 0, "elements": []}
+    assert _overlay_payload({"scene": "not-a-dict"}) == {
+        "width": 0,
+        "height": 0,
+        "elements": [],
+    }