Push a frame instead of storing and fetching it

The rate the media dtypes could carry was one frame every second or two: each
was a file on the data volume, an event on the socket, and a request back for
the bytes. This closes both halves of that, and they are one feature.

`save_artifact(..., volatile=True)` writes to a `VolatileStore` — the same
content-addressed store, in `/dev/shm`, bounded by size with the oldest falling
out (`ARTIFACT_VOLATILE_BYTES`, 48 MB under the container's raised `shm_size`).
Nothing sweeps it: a frame nobody kept is not worth walking the store to find.
`ArtifactStore.path` falls through to it, which is what lets a volatile frame be
an ordinary reference everywhere else — the dtype check, a panel's digest scope,
`load_artifact` in a node, and the widget's own fetch all work on one unchanged.
`adopt` copies one into the store when a run records it, so "returned media is
kept, emitted media is not" stays true.

The bytes then go down the flows websocket as a length-prefixed binary frame,
sent just ahead of the `message_value` naming them, so a tile has the frame when
it hears the value moved. Nothing is pushed unasked: a client names the messages
it is drawing (`{"type":"media","names":[…]}`), a panel's list is intersected
with the scope it already had, and only the newest frame per name in a batch is
sent — a client that fell behind is not handed frames it would draw over. The
tunnel relays text only, so a screen reached through a portal falls back to
fetching, which is why the rate table now has two rows.

Around the edges: the remote worker's fetch cache is bounded at last
(`FLUKSIO_ARTIFACT_CACHE_BYTES`), since content addressing means nothing in it
ever expires and a media stream fills it with chunks nothing asks for twice; a
port carrying an image draws the frame in the node panel rather than only
saying `image/png · frame.png · 1.79kB`; and an edge chip says that much instead
of a line of hash. The media screenshot stops waiting for `networkidle` — a
camera is a socket that never goes quiet, which is the point of it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YC4u66vjzW54fnHu5Juhh9

docs/reference/connector-contract.md

@@ -177,7 +177,9 @@ connector publishes a reference to it instead:
 async def poll(self) -> dict[str, Any] | None:
     jpeg = await asyncio.to_thread(self._grab)
     return {
-        "frame": self.save_artifact(jpeg, "frame.jpg", media_type="image/jpeg")
+        "frame": self.save_artifact(
+            jpeg, "frame.jpg", media_type="image/jpeg", volatile=True
+        )
     }
 ```
 
@@ -186,10 +188,18 @@ async def poll(self) -> dict[str, Any] | None:
 works once the node has started, since the store is the engine's and is handed
 over then.
 
+`volatile=True` is what a camera publishes with. The frame goes to a ring in
+memory rather than the data volume and is pushed down the websocket to
+whichever screens are drawing it, so a wall panel sees ten frames a second and
+the SD card under it is never written to. Frames last until newer ones need the
+room; leave the flag off for a reading somebody asked to keep.
+
 Each reading is a new artifact, which the poll loop publishes because its
 digest differs from the last. Set `poll_interval` to what somebody actually
-wants to look at: a frame every second or two is a glance, and live video
-belongs on the camera's own stream rather than in the graph.
+wants to look at. Through a portal the bytes are fetched rather than pushed, so
+a remote panel wants a frame every second or two; higher rates than that are
+for the local network, and full-rate video still belongs on the camera's own
+stream.
 
 ## Lifecycle