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/radio.liq

@@ -217,6 +217,23 @@ radio.on_metadata(
   end
 )
 
+# Durée et instant de départ du morceau courant, pour la règle de scrobble du
+# player (« démarré au début et écouté à ~90 % »). CAPTURÉS ICI, DANS LE THREAD
+# D'HORLOGE via un on_metadata SYNCHRONE : interroger source.duration/
+# source.elapsed depuis un handler HTTP (thread harbor) entrait en contention
+# avec l'horloge du crossfade pendant une transition et figeait l'antenne. On ne
+# touche donc plus la source hors de son horloge : /nowplaying (web.liq) se
+# contente de lire ces refs — position = temps écoulé depuis cur_started.
+cur_duration = ref(0.)
+cur_started = ref(time())
+radio.on_metadata(
+  synchronous=true,
+  fun(_) -> begin
+    cur_duration := source.duration(radio)
+    cur_started := time()
+  end
+)
+
 # --- Sortie : flux MP3 sur http://<hote>:8000/radio.mp3 ---
 output.harbor(
   %mp3(bitrate=192),