fix: a show UI that falls behind the state feed skips frames instead of freezing

[pyramation]

Summary

Chasing "the inner show UI stopped updating while the rig and the phone kept running, and only quitting the app fixed it". #114 removed one cause (a throwing send aborting the whole broadcast loop). This is the other one it left: the state feed had no per-client bound.

broadcastState() runs at 60fps for every client. ws.send buffers rather than blocks, so a client that cannot drain that fast — an embedded page the compositor isn't painting, a busy renderer — accumulates frames in its socket. It then renders state that is seconds old while messages keep arriving, so nothing looks wrong: the socket is OPEN, the stale-feed watchdog never trips, no reconnect happens, and there is no recovery short of a new connection. Exactly the reported symptom, and exactly why the outer Nova panel was unaffected — it never went through this feed.

Server. State frames are superseded by the next one, so they are now sent lossily:

export function fanoutLossy(sockets, payload, onFailure, limitBytes = FEED_BACKLOG_LIMIT_BYTES) {
  for (const socket of sockets) {
    if (socket.readyState !== OPEN_READY_STATE) continue;
    if ((socket.bufferedAmount ?? 0) > limitBytes) continue;  // skip, don't queue
    ...

Only broadcastState uses it. Commands, orientation, playlist and config-sync stay on reliable fanout — those are events, not a feed, and must not be dropped.

Client. A hidden page can't paint the feed and its timers are throttled, so it was the most reliable way to build that backlog. useSocket now closes on visibilitychange → hidden and takes a fresh connection on return (suspended distinguishes that from a break, so onclose doesn't schedule a reconnect into a hidden page). Returning to the show route therefore shows the present, not a queue.

Honest scope

This is a mechanism that provably produces the symptom, fixed from code reading — not a captured reproduction of the incident. Whether the embedded WebContentsView reports visibilityState: 'hidden' when the desktop shell hides it is the one assumption worth confirming on the show machine; if it doesn't, the server-side half still bounds the damage.

Link to Devin session: https://app.devin.ai/sessions/ec43152136134467a853ec0bbf783ea9
Requested by: @pyramation

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