stream: compute now-playing duration/position off the HTTP thread

The /nowplaying handler called source.duration/source.elapsed on the live
crossfaded source. Since it is polled every second by the web player, those
cross-thread source queries contended with the crossfade's own clock during a
transition and wedged the streaming clock — a few seconds of audio then a
frozen buffer looping on air, only ever with a listener connected.

Capture duration and the track start time in a synchronous on_metadata
callback (which runs in the clock thread, where touching the source is safe)
and have /nowplaying read those refs instead — position is just the wall-clock
time since the track started. No handler touches the source outside its clock
anymore, so the 3 s crossfade is safe again.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

stream/web.liq

@@ -95,11 +95,14 @@ harbor.http.register(
     # /scrobble pour prouver qu'il parle bien du titre à l'antenne.
     # `duration`/`position` (secondes, arrondies) permettent au player de décider
     # d'un scrobble « écouté à 90 %, démarré au début ». duration=0 si inconnue.
+    # Ces valeurs sont pré-calculées dans le thread d'horloge (voir cur_duration/
+    # cur_started dans radio.liq) : on ne doit JAMAIS appeler source.duration/
+    # source.elapsed ici, ça figerait l'antenne pendant un crossfade.
     resp.json({
       title=m["title"], artist=m["artist"], url=m["url"], origin=m["origin"],
       file=path.basename(m["filename"]),
-      duration=int_of_float(source.duration(radio)),
-      position=int_of_float(source.elapsed(radio))
+      duration=int_of_float(cur_duration()),
+      position=int_of_float(time() - cur_started())
     })
   end
 )