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.

custom_components/nvidia_shield/config_flow.py

@@ -31,6 +31,7 @@ from .const import (
     CONF_RECEIVER_ENTITY,
     CONF_RECEIVER_INPUT,
     CONF_REMOTE_COMMAND_SCRIPT,
+    CONF_SCREEN_CAPTURE_ENABLED,
     CONF_SKIP_PAIRING,
     DEFAULT_ADB_PORT,
     DEFAULT_NAME,
@@ -1438,6 +1439,13 @@ class NvidiaShieldOptionsFlow(config_entries.OptionsFlowWithReload):
                 CONF_ADB_COMMAND_ENABLED,
                 default=bool(options.get(CONF_ADB_COMMAND_ENABLED, False)),
             ): bool,
+            # t66: the screen-capture image entity. Default OFF — a capture is a
+            # full-resolution screen grab, and the standing constraint is that
+            # screen capture must never drive state.
+            vol.Optional(
+                CONF_SCREEN_CAPTURE_ENABLED,
+                default=bool(options.get(CONF_SCREEN_CAPTURE_ENABLED, False)),
+            ): bool,
             vol.Optional(CONF_JELLYFIN_ENTITY): sel["media_player"],
             vol.Optional(CONF_POWER_ENTITY): sel["power"],
         }
@@ -1917,6 +1925,7 @@ _ALL_OPTION_FIELDS = frozenset(
         CONF_ADB_STORE_APP_INFO,
         CONF_LOGCAT_EVENTS,
         CONF_ADB_COMMAND_ENABLED,
+        CONF_SCREEN_CAPTURE_ENABLED,
         CONF_JELLYFIN_ENTITY,
         CONF_POWER_ENTITY,
         CONF_RECEIVER_ENTITY,