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/tests/flow/test_workers.py

@@ -730,3 +730,84 @@ def test_cancelling_reaches_a_node_running_in_a_child(pool):
     assert pool.cancel("demo.slow"), "the parent must find a child's node"
     thread.join(timeout=10)
     assert failed
+
+
+def test_a_node_saves_a_frame_to_the_ring(tmp_path):
+    """A camera writes to memory, and the next node reads it from there.
+
+    The point of the ring being an ordinary store: node code says one word
+    more, and everything downstream — the dtype, ``load_artifact``, the widget
+    — carries on not knowing there are two of them.
+    """
+    from fluksio_worker.worker_main import ARTIFACT_VOLATILE_DIR_ENV
+
+    from fluksio.flow.artifacts import VolatileStore
+
+    store = ArtifactStore(tmp_path / "artifacts")
+    store.volatile = VolatileStore(tmp_path / "ring", limit_bytes=1_000_000)
+    pool = PythonWorkerPool(
+        python=sys.executable,
+        size=1,
+        env={
+            ARTIFACT_DIR_ENV: str(store.root),
+            ARTIFACT_VOLATILE_DIR_ENV: str(store.volatile.root),
+        },
+    )
+    pool.start()
+    try:
+        ref = pool.run(
+            "demo",
+            "frame",
+            "import fluksio\n"
+            "def process():\n"
+            "    return {'frame': fluksio.save_artifact(\n"
+            "        b'\\x89PNG' + b'p' * 64, 'f.png',\n"
+            "        media_type='image/png', volatile=True)}\n",
+            {},
+            "demo.frame",
+            timeout=10,
+        )["frame"]
+
+        assert MessageSpec(name="frame", dtype=DType.IMAGE).check(ref) is None
+        # In the ring, and nowhere near the data volume.
+        assert store.volatile.path(ref["digest"]) is not None
+        assert not (store.root / ref["digest"][7:9]).exists()
+
+        read = pool.run(
+            "demo",
+            "read",
+            "import fluksio\n"
+            "def process(frame):\n"
+            "    with open(fluksio.load_artifact(frame), 'rb') as f:\n"
+            "        return {'size': len(f.read())}\n",
+            {"frame": ref},
+            "demo.read",
+            timeout=10,
+        )
+        assert read == {"size": 68}
+    finally:
+        pool.stop()
+
+
+def test_the_fetch_cache_is_bounded(tmp_path):
+    """A worker downloading a media stream fills its cache with chunks nothing
+    will ask for twice, and content addressing means nothing ever expires."""
+    import os
+
+    from fluksio_worker.worker_main import ARTIFACT_CACHE_BYTES_ENV, _trim_cache
+
+    cache = tmp_path / "cache"
+    cache.mkdir()
+    for index in range(5):
+        entry = cache / f"chunk{index}"
+        entry.write_bytes(b"c" * 1000)
+        os.utime(entry, (index, index))
+
+    os.environ[ARTIFACT_CACHE_BYTES_ENV] = "3000"
+    try:
+        _trim_cache(str(cache))
+    finally:
+        del os.environ[ARTIFACT_CACHE_BYTES_ENV]
+
+    left = sorted(entry.name for entry in cache.iterdir())
+    assert left == ["chunk2", "chunk3", "chunk4"]