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).

tests/test_ui_parser.py

@@ -0,0 +1,81 @@
+from __future__ import annotations
+
+from perception.ui_parser import parse_ui_tree
+
+
+def test_ios_page_source_reports_enabled_only() -> None:
+    xml = """
+    <AppiumAUT x="0" y="0" width="100" height="200">
+      <XCUIElementTypeButton name="Submit" label="Submit" enabled="false"
+                             visible="true" x="1" y="2" width="4" height="4" />
+    </AppiumAUT>
+    """
+
+    elements = parse_ui_tree(xml)
+
+    button = next(element for element in elements if element.type == "button")
+    assert button.enabled is False
+    # iOS page_source never emits these, so they must stay unknown (None),
+    # not be defaulted to False.
+    assert button.clickable is None
+    assert button.selected is None
+    assert button.checked is None
+    assert button.focused is None
+
+
+def test_android_page_source_reports_full_interaction_state() -> None:
+    xml = """
+    <hierarchy>
+      <node class="android.widget.CheckBox" text="Remember me"
+            enabled="true" clickable="true" selected="false" checked="true"
+            focused="false" bounds="[10,20][60,50]" />
+    </hierarchy>
+    """
+
+    elements = parse_ui_tree(xml)
+
+    assert len(elements) == 1
+    checkbox = elements[0]
+    assert checkbox.enabled is True
+    assert checkbox.clickable is True
+    assert checkbox.selected is False
+    assert checkbox.checked is True
+    assert checkbox.focused is False
+
+
+def test_dict_tree_passes_through_native_bool_state() -> None:
+    tree = {
+        "type": "button",
+        "text": "Send",
+        "x": 0,
+        "y": 0,
+        "width": 10,
+        "height": 10,
+        "enabled": True,
+        "clickable": False,
+    }
+
+    elements = parse_ui_tree(tree)
+
+    assert len(elements) == 1
+    assert elements[0].enabled is True
+    assert elements[0].clickable is False
+    assert elements[0].selected is None
+
+
+def test_to_dict_omits_unknown_state_fields_but_keeps_reported_false() -> None:
+    xml = """
+    <AppiumAUT x="0" y="0" width="10" height="10">
+      <XCUIElementTypeButton name="Submit" enabled="false" x="0" y="0" width="4" height="4" />
+    </AppiumAUT>
+    """
+
+    elements = parse_ui_tree(xml)
+    element = next(item for item in elements if item.type == "button")
+    data = element.to_dict()
+
+    assert data["enabled"] is False
+    assert "clickable" not in data
+    assert "selected" not in data
+    assert "checked" not in data
+    assert "focused" not in data