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

frontend/scripts/capture-screenshots.mjs

@@ -199,13 +199,18 @@ async function captureDashboards(page, dir) {
  *
  * Skipped unless the media example is seeded (root `make seed-example-media`),
  * since it is the one shot that needs a source of frames. The bytes arrive as
- * a blob — the tile fetches them with the session's credential, which no `img`
- * could carry on its own — so a `blob:` source is the proof the whole path ran
- * rather than that a picture is merely present.
+ * a blob — pushed down the socket, or fetched with the session's credential,
+ * which no `img` could carry on its own — so a `blob:` source is the proof the
+ * whole path ran rather than that a picture is merely present.
+ *
+ * The one page that cannot wait for `networkidle`: a camera publishing several
+ * frames a second is a socket that never goes quiet, which is the point of it.
+ * The blob source below is a stronger wait anyway — it says a frame arrived,
+ * where idleness only ever said the page stopped asking.
  */
 async function captureMedia(page, dir) {
   const answer = await page.goto(`${APP_URL}/view/camera`, {
-    waitUntil: "networkidle",
+    waitUntil: "domcontentloaded",
   })
   if (!answer?.ok()) return