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.

storage/timeline.py

@@ -18,6 +18,7 @@ class TimelineRecord:
     result: dict[str, Any]
     timestamp: str
     ocr_results: list[dict[str, Any]] = field(default_factory=list)
+    ui_tree_results: list[dict[str, Any]] = field(default_factory=list)
     before_screenshot_path: str | None = None
     after_screenshot_path: str | None = None
     screenshot_path: str | None = None
@@ -39,12 +40,14 @@ class Timeline:
         before_screenshot: bytes | None = None,
         after_screenshot: bytes | None = None,
         ocr_results: list[dict[str, Any]] | None = None,
+        ui_tree_results: list[dict[str, Any]] | None = None,
     ) -> TimelineRecord:
         index = len(self.read(task_id)) + 1
         resolved_after_screenshot = (
             after_screenshot if after_screenshot is not None else screenshot
         )
         resolved_ocr_results = list(ocr_results or [])
+        resolved_ui_tree_results = list(ui_tree_results or [])
         record = {
             "index": index,
             "scene": scene,
@@ -52,6 +55,7 @@ class Timeline:
             "tool_call": tool_call,
             "result": result,
             "ocr_results": resolved_ocr_results,
+            "ui_tree_results": resolved_ui_tree_results,
             "timestamp": datetime.now().astimezone().isoformat(),
         }
         paths = self.artifact_store.write_step(
@@ -69,6 +73,7 @@ class Timeline:
             result=result,
             timestamp=record["timestamp"],
             ocr_results=resolved_ocr_results,
+            ui_tree_results=resolved_ui_tree_results,
             before_screenshot_path=paths["before_screenshot_path"],
             after_screenshot_path=paths["after_screenshot_path"],
             screenshot_path=paths["screenshot_path"],