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/nativeTransport.js

@@ -57,7 +57,7 @@ export function createNativeTransport(dispatch) {
         /** Accept a native lease and hand its clip to the preview player.
          * `startPaused` maps to the backend's desired-paused latch, so a
          * parked open can never leak audio (Phase 0.4). */
-        async load(lease, { sessionId, generation, sessionKey }, { startPaused = false } = {}) {
+        async load(lease, { sessionId, generation, sessionKey }, { startPaused = false, crossfadeMs = 0, startAtMs = 0 } = {}) {
             if (disposed) throw new Error("transport disposed");
             await ensureSubscribed();
             if (!isValidLease(lease) || lease.transport !== "native") {
@@ -67,7 +67,14 @@ export function createNativeTransport(dispatch) {
                 return;
             }
             ctx = { sessionId, generation, sessionKey };
-            await api.previewLoad(lease.opaqueLeaseId, startPaused);
+            // startAtMs rides into the DECODER (session resume): the clip
+            // comes up already positioned, no post-load seek race.
+            await api.previewLoad(
+                lease.opaqueLeaseId,
+                startPaused,
+                crossfadeMs,
+                startAtMs > 0 ? startAtMs / 1000 : null
+            );
         },
         async play() {
             if (!ctx) return;
@@ -84,6 +91,15 @@ export function createNativeTransport(dispatch) {
         async setVolume(v) {
             await api.previewControl("volume", v);
         },
+        /** Crossfade out-half: ramp the current clip to silence instead of
+         * cutting it. ctx is deliberately KEPT — the tail's events carry the
+         * superseded generation and die at the reducer, and the next load
+         * will restamp; clearing ctx here would also mute a failure event
+         * racing the fade. */
+        async fadeOut(ms) {
+            if (!ctx) return;
+            await api.previewControl("fade_out", ms);
+        },
         async stop() {
             ctx = null; // events from the dying clip stamp nothing
             await api.previewControl("stop");