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/core/config.py

@@ -150,6 +150,17 @@ class Settings(BaseSettings):
     # Storing bytes and recording the reference are two steps; this is the
     # window between them.
     ARTIFACT_GC_GRACE_S: int = 3600
+    #: Where frames a flow only shows live are held: memory rather than the
+    #: data volume, so a camera at ten frames a second is not writing to an SD
+    #: card. Empty works it out — a directory under `/dev/shm` named for this
+    #: data directory, so two instances on one host do not trim each other —
+    #: and falls back to the temporary directory where there is no `/dev/shm`.
+    ARTIFACT_VOLATILE_DIR: Path | None = None
+    #: How much the volatile ring holds before the oldest frames fall out of
+    #: it; 0 turns it off, and a volatile save then lands in the durable store
+    #: like any other. Under Docker's default 64 MB `/dev/shm`: raise
+    #: `shm_size` with it.
+    ARTIFACT_VOLATILE_BYTES: int = 48 * 1024 * 1024
     # Without a Redis host the engine keeps its state in memory.
     REDIS_HOST: str | None = None
     REDIS_PORT: int = 6379