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

backend/fluksio/flow/connector.py

@@ -141,6 +141,7 @@ class ConnectorNode(Node):
         data: bytes,
         name: str = "",
         media_type: str = "application/octet-stream",
+        volatile: bool = False,
     ) -> dict[str, Any]:
         """Store bytes and return the reference to publish on a media port.
 
@@ -149,6 +150,11 @@ class ConnectorNode(Node):
         store and the reference names them, which is what an ``image``,
         ``audio`` or ``video`` port carries.
 
+        ``volatile`` is what a camera publishes with: the bytes go to a ring
+        held in memory rather than to the data volume, are pushed to whatever
+        screen is watching, and last seconds. Use it for a frame; leave it off
+        for a recording somebody asked to keep.
+
         Only available once the node has started — the store belongs to the
         engine, and is handed over then.
         """
@@ -156,7 +162,9 @@ class ConnectorNode(Node):
             raise RuntimeError(
                 "no artifact store: a connector can only save bytes once it has started"
             )
-        return self._artifacts.put([data], name=name, media_type=media_type)
+        return self._artifacts.put(
+            [data], name=name, media_type=media_type, volatile=volatile
+        )
 
     async def start(self, app: FastAPI | None = None) -> None:
         self._artifacts = getattr(app.state, "artifact_store", None) if app else None