iPhone: step probes + full hls.js error stream to server

The play-probe now lands (iPhone: iOS 18.7 Safari, hls.js path, MSE ok),
but the /stream XHR never reaches the server on the failing attempts —
impossible under the known code path. Instrument each step
(new-hls-ok / loadsource-called / attach-ok) and log EVERY hls.js error
(fatal+non-fatal, with retry counter) so the client timeline is visible
server-side. Also fixed JSV constant drift (was stuck at 10).
events endpoint: accept hls-err, forward step/fatal/details/buffer/retry/http.

static/index.html

@@ -6,7 +6,7 @@
 <title>ani-web</title>
 <link rel="icon" href="data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 64 64'%3E%3Crect width='64' height='64' rx='14' fill='%23f97316'/%3E%3Ctext x='32' y='45' font-family='sans-serif' font-size='42' font-weight='700' text-anchor='middle' fill='%230e0f13'%3Ea%3C/text%3E%3C/svg%3E">
 
-<link rel="stylesheet" href="style.css?v=11">
+<link rel="stylesheet" href="style.css?v=12">
 </head>
 <body>
 <header>
@@ -30,6 +30,6 @@
      max-age=300 in browser AND possibly in your HTTPS proxy, and a stale
      copy is exactly how users keep seeing an old UI after a deploy -->
 <script src="hls.min.js"></script>
-<script src="app.js?v=11"></script>
+<script src="app.js?v=12"></script>
 </body>
 </html>