feat(perception): surface accessibility interaction state on UI-tree elements

SceneElement gains enabled/clickable/selected/checked/focused (bool | None),
populated from the literal attributes Appium's XCUITest and UiAutomator2
page_source already emit (iOS: enabled only; Android: all five). None means
"not reported by this platform", not false. to_dict() omits unset fields to
keep the LLM-facing scene JSON compact; planner_prompts.py documents the new
fields so the AI planner knows how to use them (e.g. don't tap disabled
elements, use selected/checked to judge whether a toggle already matches the
goal).

core/models.py

@@ -71,6 +71,9 @@ class Device:
         }
 
 
+_STATE_FIELDS = ("enabled", "clickable", "selected", "checked", "focused")
+
+
 @dataclass
 class SceneElement:
     id: str
@@ -79,6 +82,13 @@ class SceneElement:
     text: str | None = None
     confidence: float | None = None
     source: str | None = None
+    # Accessibility-tree interaction state, when the platform reports it.
+    # None means "not reported by this platform/element", not "false".
+    enabled: bool | None = None
+    clickable: bool | None = None
+    selected: bool | None = None
+    checked: bool | None = None
+    focused: bool | None = None
 
     @property
     def center(self) -> tuple[float, float]:
@@ -94,6 +104,10 @@ class SceneElement:
         }
         if self.source:
             data["source"] = self.source
+        for field_name in _STATE_FIELDS:
+            value = getattr(self, field_name)
+            if value is not None:
+                data[field_name] = value
         return data
 
     @classmethod
@@ -105,6 +119,11 @@ class SceneElement:
             bounds=Bounds.from_dict(data["bounds"]),
             confidence=data.get("confidence"),
             source=data.get("source"),
+            enabled=data.get("enabled"),
+            clickable=data.get("clickable"),
+            selected=data.get("selected"),
+            checked=data.get("checked"),
+            focused=data.get("focused"),
         )