Cameras on devices (openpanel C1): frames in memory, a binary uplink, demand

- Volatile frames are held in memory with a byte-bounded queue instead of
  the /dev/shm ring scanned on every put; workers keep writing to the
  directory, which is trimmed on a timer. A volatile upload no longer spools
  to disk. ARTIFACT_VOLATILE_BYTES=0 turns the ring off.
- A device sends camera frames as binary messages on its socket, held and
  published in one step and credited to its device node; refused frames are
  logged and dropped. Its snapshot carries demand and the uplink (binary, or
  http through the portal).
- One demand registry for every socket's wanted media, the tunnel's
  included, plus what running flows read, mapped onto on-demand ports and
  published as device_demand; at most 8 media names per socket.
- Per-message deflate off in serve and dev; a device may fetch the last 8
  digests of a message; published dashboards cached per store revision; a
  revoked device socket is closed with 1008.
- Media tiles want bytes only while on screen; the Devices tab previews a
  wired camera; the device node warns when a flow reads an on-demand port.

backend/tests/flow/test_workers.py

@@ -790,7 +790,7 @@ def test_a_node_saves_a_frame_to_the_ring(tmp_path):
 
         assert MessageSpec(name="frame", dtype=DType.IMAGE).check(ref) is None
         # In the ring, and nowhere near the data volume.
-        assert store.volatile.path(ref["digest"]) is not None
+        assert store.volatile.has(ref["digest"])
         assert not (store.root / ref["digest"][7:9]).exists()
 
         read = pool.run(