Fix P1/A1 camera black screen from fan-out churn on single-connection cams (#2521)

Chamber-image printers came up black on load and only recovered ~20 min
later. Two causes: (1) late fan-out subscribers got an empty queue and
waited for the next frame, so the browser never fired onLoad and the
stall-detector reconnect-looped, churning short-lived viewers; (2) the
churn reopened the port-6000 socket before the old one closed, so the
printer fed an orphaned socket until its TCP keepalive reaped it.

Prime late subscribers with the last pumped frame; make a replacement
broadcaster's pump wait for the predecessor's socket to close before
dialing (bounded 10s); require two consecutive stalled reads before the
frontend reconnects.

static/index.html

@@ -26,7 +26,7 @@
 
     <!-- Splash screens for iOS -->
     <link rel="apple-touch-startup-image" href="/img/android-chrome-512x512.png" />
-    <script type="module" crossorigin src="/assets/index-BC5UbFyP.js"></script>
+    <script type="module" crossorigin src="/assets/index-aVVJQik4.js"></script>
     <link rel="stylesheet" crossorigin href="/assets/index-blSspT6K.css">
   </head>
   <body>