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/api/routes/artifacts.py

@@ -80,6 +80,7 @@ async def put_artifact(
     request: Request,
     name: str = Query(default=""),
     media_type: str = Query(default=""),
+    volatile: bool = Query(default=False),
 ) -> Any:
     """Store the request body and answer with the reference to it.
 
@@ -87,6 +88,10 @@ async def put_artifact(
     legitimate a body here as a checkpoint, and neither should have to fit in
     memory twice. Capped, because nothing else here was: any account, and any
     worker credential, could otherwise fill the data volume.
+
+    ``volatile`` puts it in the ring instead of the store — a frame a screen is
+    watching now, which the oldest of falls out of memory rather than being
+    kept. A worker on another host publishing a camera sends this.
     """
     store = _store(request)
     cap = settings.MAX_ARTIFACT_BYTES
@@ -112,7 +117,7 @@ async def put_artifact(
                 # as long as the upload lasts.
                 await run_in_threadpool(handle.write, chunk)
         return await run_in_threadpool(
-            store.put_file, Path(handle.name), media_type, name
+            store.put_file, Path(handle.name), media_type, name, volatile
         )
     finally:
         Path(handle.name).unlink(missing_ok=True)