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-tauri/src/lib.rs

@@ -1,8 +1,10 @@
 mod commands;
 mod config;
 mod cover_logo;
+mod diagnostics;
 mod hero_crop;
 mod hero_detector;
+mod loudness;
 mod pipeline;
 mod playback;
 mod preview;
@@ -31,6 +33,8 @@ pub fn run() {
             playback::init(app.handle());
             playback::prune_cache(app.handle());
             preview::init(app.handle());
+            loudness::init(app.handle());
+            diagnostics::init(app.handle());
             if let Err(e) = pipeline::init(app.handle()) {
                 log::error!("library init failed: {e}");
             }
@@ -108,6 +112,9 @@ pub fn run() {
             preview::preview_control,
             preview::preview_dispose,
             preview::player_pause_acked,
+            preview::player_fade_pause_acked,
+            preview::player_fade_resume,
+            diagnostics::debug_stats,
         ])
         .run({
             #[allow(unused_mut)]