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.

apps/device-host-agent/host_agent/web/app.py

@@ -94,20 +94,30 @@ def _ocr_results(record: dict[str, Any]) -> list[dict[str, Any]]:
 
 
 def _ui_tree_nodes(record: dict[str, Any]) -> list[dict[str, Any]]:
-    tool_call = record.get("tool_call")
-    if not isinstance(tool_call, dict):
-        return []
-    if tool_call.get("action") not in {"get_ui_tree", "ui_tree"}:
-        return []
-    step_result = record.get("result")
-    if not isinstance(step_result, dict):
-        return []
-    raw_nodes = step_result.get("result")
+    raw_nodes = record.get("ui_tree_results")
     if not isinstance(raw_nodes, list):
         return []
     return [node for node in raw_nodes if isinstance(node, dict)]
 
 
+def _overlay_payload(record: dict[str, Any]) -> dict[str, Any]:
+    """Combined perception elements + screen size for client-side bounding-box
+    overlay and action-effect rendering on the before-screenshot.
+    """
+    scene = record.get("scene")
+    screen = scene.get("screen") if isinstance(scene, dict) else None
+    width = screen.get("width") if isinstance(screen, dict) else None
+    height = screen.get("height") if isinstance(screen, dict) else None
+    elements = scene.get("elements") if isinstance(scene, dict) else None
+    return {
+        "width": width if isinstance(width, (int, float)) else 0,
+        "height": height if isinstance(height, (int, float)) else 0,
+        "elements": [element for element in elements if isinstance(element, dict)]
+        if isinstance(elements, list)
+        else [],
+    }
+
+
 def _timeline_step_context(record: dict[str, Any]) -> dict[str, Any]:
     tool_call = record.get("tool_call")
     result = record.get("result")
@@ -126,6 +136,7 @@ def _timeline_step_context(record: dict[str, Any]) -> dict[str, Any]:
         or _screenshot_data_uri(record),
         "ocr_results": _ocr_results(record),
         "ui_tree_nodes": _ui_tree_nodes(record),
+        "overlay": _overlay_payload(record),
     }