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/App.svelte

@@ -6,6 +6,7 @@
     import ArtworkViewer from "./components/ArtworkViewer.svelte";
     import LoginModal from "./components/LoginModal.svelte";
     import PreviewOverlay from "./components/PreviewOverlay.svelte";
+    import DebugOverlay from "./components/DebugOverlay.svelte";
     import { previewUi } from "./lib/preview/previewSession.svelte.js";
     import Home from "./views/Home.svelte";
     import Search from "./views/Search.svelte";
@@ -52,21 +53,23 @@
     class:preview-open={previewUi.open}
 >
     <TitleBar />
-    <!-- Previews take over the whole window (decision 2026-07-22): while the
-         modal is open the page and the player bar are inert/not rendered —
-         there is deliberately NO UI path to command main audio behind the
-         focus coordinator's back. The titlebar stays live for window chrome. -->
+    <!-- While the preview modal is open the page is inert and the flanking
+         panels + player bar are pointer-inert under the scrim (user decision
+         2026-07-23: the bar stays VISIBLE, dimmed, like Spotify — un-rendering
+         it left a black hole in the grid's bottom row). There is still no UI
+         path to command main audio behind the focus coordinator's back: the
+         scrim intercepts pointer input and the dialog traps the keyboard.
+         The titlebar stays live for window chrome. -->
     <Sidebar />
     <main class="content" use:floatScroll inert={previewUi.open || undefined}>
         <CurrentView />
     </main>
     <NowPlaying />
-    {#if !previewUi.open}
-        <PlayerBar />
-    {/if}
+    <PlayerBar />
     <PreviewOverlay />
     <ArtworkViewer />
     <LoginModal />
+    <DebugOverlay />
 </div>
 
 <style>
@@ -74,10 +77,11 @@
        no separate chrome color, the panels float beneath it. */
     /* While the preview modal is open: keyboard is contained by the dialog's
        focus trap and aria-modal, the content pane is `inert`, and the flanking
-       panels stop taking pointer input here. (The player bar is simply not
-       rendered — see the markup note above.) */
+       panels + player bar stop taking pointer input here — the bar stays
+       visible (dimmed under the scrim) but cannot command main audio. */
     .app.preview-open :global(.sidebar),
-    .app.preview-open :global(.nowplaying) {
+    .app.preview-open :global(.nowplaying),
+    .app.preview-open :global(.player) {
         pointer-events: none;
     }