feat(image): opt-in screen capture as an image entity, binary-safe over ADB

The user asked for "an image entity of the device screen capture? Can that be
added". It is added as `image.<entry>_screen_capture`, and the three boundaries
are the feature:

- it NEVER drives state — the state is the capture timestamp, and the capture
  path reads the ADB poller and nothing else (pinned by a manager that raises on
  any other attribute, and by a real-HA run whose recorded state is the
  timestamp with no bytes anywhere in the attributes);
- it is OPT-IN and OFF by default (`screen_capture_enabled`, Main setup row) —
  with the option absent `async_setup_entry` adds no entity at all;
- it NEVER polls — HA's own `should_poll = False` for image entities is kept, so
  only an image fetch or `homeassistant.update_entity` captures; concurrent
  fetches share one shell and a capture is reused for 5 s.

BINARY-SAFE CAPTURE: `screencap -p | base64` over the existing shell, decoded
locally with `base64.b64decode` and gated on the PNG signature AND the IHDR
chunk. `AdbDevice.shell()` returns `str`, so raw PNG bytes cannot travel through
it; the alternative (a device temp file + the library's file-read path) needs the
raw AdbDevice and writes to the Shield's shared storage, where a crash between
write and delete leaves a screenshot behind. The base64 route leaves no
device-side artifact — nothing to clean up — at the cost of ~33% more bytes on
the wire.

HONEST FAILURES: ADB off / ADB down / a capture that returned no PNG / a
transport that raised all make the entity UNAVAILABLE with `capture_status`
(`adb_disabled` / `adb_disconnected` / `capture_failed`) — never a blank or
zero-byte image. `image_last_updated` moves only on success.

Tests: 25 new cases (a real 2x1 PNG fixture on the wire — bytes, never a str;
every non-PNG answer rejected; disconnected/failed/recovery paths; capture reuse,
coalescing and the expiry window; opt-in default OFF and on) plus 4 config-flow
cases (schema default False, off stays off, on is stored, and it can be turned
back off). The stub harness gains the image platform and `util.dt` faces, the
real-HA entity matrix gains this class, and the drift sweep's coverage test
demanded exactly that. README documents the cost, the opt-in and the boundary.

tests/test_config_flow_ux.py

@@ -47,6 +47,7 @@ from nsh.const import (
     CONF_POWER_ENTITY,
     CONF_PROTOCOL_ENABLED,
     CONF_REMOTE_COMMAND_SCRIPT,
+    CONF_SCREEN_CAPTURE_ENABLED,
     CONF_SKIP_PAIRING,
     DEFAULT_ADB_PORT,
     SCRIPT_COMMANDS,
@@ -249,7 +250,7 @@ def _install_stubs() -> None:
     ha_const.EVENT_HOMEASSISTANT_STOP = "homeassistant_stop"
     ha_const.Platform = types.SimpleNamespace(
         MEDIA_PLAYER="media_player", SENSOR="sensor", REMOTE="remote",
-        BUTTON="button",
+        BUTTON="button", IMAGE="image",
     )
     sys.modules["homeassistant.const"] = ha_const
 
@@ -1603,6 +1604,7 @@ OPTIONS_SUBSTEP_FIELDS = {
         CONF_POWER_ENTITY,
         CONF_LOGCAT_EVENTS,
         CONF_ADB_COMMAND_ENABLED,
+        CONF_SCREEN_CAPTURE_ENABLED,
     },
     "receiver": {"receiver_entity", "receiver_input"},
     # t64: the apps table lives in its OWN step now (the Apps sub-screen is a menu
@@ -2765,3 +2767,44 @@ class TestSetupMenu:
         assert flow._host() == "10.0.0.77"
         assert result["options"].get("receiver_entity") == "switch.soundbar"
         assert result["options"].get("receiver_input") == "HDMI 1"
+
+
+class TestScreenCaptureOption:
+    """t66: the screen-capture image entity is opt-in, and OFF is the default."""
+
+    async def test_the_connection_screen_offers_it_and_defaults_off(self, tmp_path):
+        """Default OFF at the SCHEMA level — the form the user is shown."""
+        flow = _options_flow(tmp_path)
+
+        result = await _options_open(flow, "connection")
+
+        defaults = vol_mirror.defaults_by_key(result["data_schema"])
+        assert defaults[CONF_SCREEN_CAPTURE_ENABLED] is False
+
+    async def test_an_untouched_toggle_stays_off(self, tmp_path):
+        """A save that never renders the screen-capture row must not invent it."""
+        flow = _options_flow(tmp_path)
+
+        result = await _options_submit(flow, {CONF_HOST: "10.0.0.9"}, save=True)
+
+        assert result["type"] == "create_entry"
+        assert not result["data"].get(CONF_SCREEN_CAPTURE_ENABLED)
+
+    async def test_switching_it_on_is_stored(self, tmp_path):
+        flow = _options_flow(tmp_path)
+
+        result = await _options_submit(
+            flow, {CONF_SCREEN_CAPTURE_ENABLED: True}, save=True
+        )
+
+        assert result["type"] == "create_entry"
+        assert result["data"][CONF_SCREEN_CAPTURE_ENABLED] is True
+
+    async def test_it_can_be_switched_back_off(self, tmp_path):
+        flow = _options_flow(tmp_path, {CONF_SCREEN_CAPTURE_ENABLED: True})
+
+        result = await _options_submit(
+            flow, {CONF_SCREEN_CAPTURE_ENABLED: False}, save=True
+        )
+
+        assert result["data"][CONF_SCREEN_CAPTURE_ENABLED] is False