fix: preview crossfade no longer strands the main volume (silent playback)

Clicking a track could play silently: the store showed status=playing with
the position advancing and no error, but the sink was at volume 0. The
open/close crossfade ramped the main player's USER VOLUME as its fade
channel and left it at 0 on an ack timeout (or when a track change
suppressed the close resume). User volume persists across tracks and a new
Play doesn't reset it, so every later click played at 0 while the slider
still read 0.7.

Give the main player a separate fade_gain channel (Cmd::SetFadeGain, sink =
volume * norm * fade_gain), reset to 1 by every Play/Stop. The open/close
fades ramp fade_gain instead of set_volume (their volume arg is gone, JS
callers too), so the user volume is never touched -- store and backend can't
desync -- and a clicked track always resets fade_gain to 1, so it can never
be stranded silent. fade_resume lands exactly at 1. Player::fade_gain()
mirrors it into a report atomic; debug overlay shows main fade + this proved
the fix live (open 1->0, close 0->1, new track 1.0, volume steady at 0.7).

39 app + 12 player + 85 JS tests pass.

src/lib/preview/focusBridge.js

@@ -105,14 +105,15 @@ export function createProductionFocus({ readPlayer, readPlaybackGeneration, resu
     // Deferred open crossfade: fade the main track out NOW, synchronised with
     // the preview's fade-in. Idempotent, and only fades if the main is
     // actually sounding (guards a commitment that leaked past a handoff or an
-    // idle-main open). Fresh volume read at fire time.
+    // idle-main open). The fade rides a separate fade-gain channel — the user
+    // volume is never touched.
     coordinator.beginMainSilence = () => {
         if (!mainSilenceArmed) return;
         mainSilenceArmed = false;
         const p = readPlayer();
         const audible = p.status === "playing" || p.status === "buffering";
         if (!audible) return;
-        void api.playerFadePauseAcked(CROSSFADE_MS, p.muted ? 0 : (p.volume ?? 0.7));
+        void api.playerFadePauseAcked(CROSSFADE_MS);
     };
 
     return { coordinator, gateway };