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/views/Settings.svelte

@@ -38,6 +38,12 @@
         }
     }
 
+    async function setVolumeNormalization(next) {
+        if (next !== settings.volume_normalization) {
+            await commit({ volumeNormalization: next });
+        }
+    }
+
     async function chooseDownloadDir() {
         const dir = await api.pickFolder();
         if (dir) await commit({ downloadDir: dir });
@@ -226,6 +232,21 @@
                     {/each}
                 </select>
             </div>
+            <div class="row">
+                <div class="label">
+                    <div class="name">Normalize volume</div>
+                    <div class="hint">Plays every track at roughly the same loudness (−14 LUFS, like Spotify), so quiet and loud masters stop needing the volume knob. Streams are measured during their first play; boosts never clip.</div>
+                </div>
+                <select
+                    class="device-select"
+                    value={settings.volume_normalization ? "on" : "off"}
+                    disabled={saving}
+                    onchange={(e) => setVolumeNormalization(e.currentTarget.value === "on")}
+                >
+                    <option value="on">On (recommended)</option>
+                    <option value="off">Off</option>
+                </select>
+            </div>
             <p class="muted small">Changes apply immediately and keep the current track near the same position.</p>
         </section>