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.

crates/metadata/src/lib.rs

@@ -6360,19 +6360,15 @@ pub async fn fetch_preview_media(url: &str) -> Result<PreviewMediaResponse, Prev
     if !preview_url_allowed(url) {
         return Err(PreviewFetchError::DisallowedUrl);
     }
-    let resp = preview_media_client()
-        .get(url)
-        .send()
-        .await
-        .map_err(|e| {
-            // A redirect rejected by the policy surfaces as a generic
-            // reqwest error; classify it as the policy violation it is.
-            if e.is_redirect() {
-                PreviewFetchError::DisallowedUrl
-            } else {
-                PreviewFetchError::Network
-            }
-        })?;
+    let resp = preview_media_client().get(url).send().await.map_err(|e| {
+        // A redirect rejected by the policy surfaces as a generic
+        // reqwest error; classify it as the policy violation it is.
+        if e.is_redirect() {
+            PreviewFetchError::DisallowedUrl
+        } else {
+            PreviewFetchError::Network
+        }
+    })?;
     let status = resp.status();
     if !status.is_success() {
         return Err(PreviewFetchError::Status(status.as_u16()));