feat: crossfade preview suite, loudness normalization, debug overlay

Preview modal audio, five rounds:
- True track-change crossfade: scheduleSettle no longer silences the
  outgoing clip at t=0; preview_load's equal-power fade owns both ramps,
  gated on the incoming voice's first Playing event (audible_marks), with
  failure/overstay fallbacks. CROSSFADE_MS = 340 — locked to the reel's
  card-slide so audio and visuals land together.
- Two-neighbour lease prefetch + a 15-min on-disk clip byte-cache
  (recent_clips): steps in BOTH directions crossfade in ~15 ms; revisits
  never re-download.
- Adaptive settle (isolated steps fire immediately, bursts coalesce) and
  anticipatory auto-advance (step at clip end minus 400 ms) so auto-play
  crossfades over the tail instead of gapping.
- Session resume memory: revisited tracks continue where they left off
  (decoder-level start offset via play_local_from/resume_at); finished
  tracks restart from the top.
- Open/close crossfades with the MAIN player: fade-pause with the
  desired-paused latch landing at ramp end, fade-resume from the paused
  position; MAIN_FADE_GEN guards; focus handshake relaxation documented
  in focusBridge.

Volume normalization (Settings > Playback, default on):
- Dependency-free BS.1770/EBU R128 meter in the player crate (K-weighting,
  400 ms gated blocks, incremental) + analyzer that reads the same growing
  file playback streams from — first plays are measured at download speed.
- Gain layered under user volume; audible start GATED on a preliminary
  measurement (10 s of audio or 800 ms budget) so the level is right from
  the first sample and never changes mid-track; prelim boosts capped 3 dB,
  peak-aware clipping guard, persistent loudness-cache.json.
- Gapless boundaries get their own sample-accurate gain via
  SetNextNormGainDb, fed by preload analysis.

Debug overlay (Ctrl+Shift+D): process-tree CPU / PDH GPU / working-set +
private-commit RAM / network rates / FPS / current-track download progress
read from the backend's own StreamProgress / live normalization state /
preview session phase. Polls only while visible.

131 JS + player + app tests green; every path verified live over CDP.

src/lib/preview/previewSession.svelte.js

@@ -145,19 +145,25 @@ export async function closePreview() {
         demoTicker = null;
     }
     if (!service) return { restored: false };
-    const out = await service.close();
-    unsubscribe?.();
-    unsubscribe = null;
+    // Detach first (re-entrancy: a second Escape lands on the early return),
+    // and hide the modal BEFORE awaiting the close — service.close() now
+    // holds the line for the close-crossfade tail (~CROSSFADE_MS), and the
+    // visual out-transition must not wait on audio. The snapshot stays until
+    // the close resolves so the dialog has content through its exit.
+    const svc = service;
     service = null;
     previewUi.open = false;
-    previewUi.snapshot = null;
-    previewUi.focusWaiting = false;
-    previewUi.playFull = null;
     const back = previewUi.returnFocusTo;
     previewUi.returnFocusTo = null;
     if (back && typeof back.focus === "function" && back.isConnected) {
         back.focus();
     }
+    const out = await svc.close();
+    unsubscribe?.();
+    unsubscribe = null;
+    previewUi.snapshot = null;
+    previewUi.focusWaiting = false;
+    previewUi.playFull = null;
     return out;
 }