Phase 2A+2B: native preview backend and renderer glue

Rust (2A):
- metadata: deezer_track_uncached (fresh mint, cache-updating, paced) and
  fetch_preview_media — dedicated client whose custom redirect policy
  re-validates every hop against the media allowlist; bounded body read
  (cap enforced on received bytes, not Content-Length's promise); Date
  header captured for provider-clock lease math. httpdate dep added.
- src-tauri/preview.rs: second Player instance + soulstream:preview event
  channel; preview_resolve (fresh mint → host gate → MIME gate → ID3v2
  shell strip → MPEG sync check → clip file + opaque lease id → safe_for_ms
  mirroring lease.js); preview_load with start_paused via the
  desired-paused latch; preview_control/dispose; player_pause_acked
  exposing the Phase 0.4 ack on the MAIN player. 9 unit tests.

JS (2B):
- api.js: preview command surface (opaque lease ids only).
- nativeTransport.js: stamps {sessionId, generation, sessionKey} onto
  backend events; cleared context makes stragglers dispatch nothing.
- productionResolver.js: DTO → MediaLease, failure classes preserved.
- focusBridge.js: production coordinator binding with the documented
  renderer-side generation invariant (buffering counter + gateway).
- service.js: the orchestrator — settle window, resolve→load→play under
  focus, single capped retry, one forward lease prefetch with safe-window
  expiry, stale re-checks before every transport command, close/handoff.
  The service constructs its transport around its own dispatch — the
  factory signature exists because a pre-built transport silently loses
  the event wiring (caught by test).

70 JS tests passing (npm test); metadata 46 + app preview 9 in cargo.

src-tauri/src/lib.rs

@@ -5,6 +5,7 @@ mod hero_crop;
 mod hero_detector;
 mod pipeline;
 mod playback;
+mod preview;
 mod state;
 
 #[cfg_attr(mobile, tauri::mobile_entry_point)]
@@ -29,6 +30,7 @@ pub fn run() {
             hero_detector::warm_up(app.handle().clone());
             playback::init(app.handle());
             playback::prune_cache(app.handle());
+            preview::init(app.handle());
             if let Err(e) = pipeline::init(app.handle()) {
                 log::error!("library init failed: {e}");
             }
@@ -101,6 +103,11 @@ pub fn run() {
             commands::preload_stream,
             commands::preload_cached_stream,
             commands::stream_waveform,
+            preview::preview_resolve,
+            preview::preview_load,
+            preview::preview_control,
+            preview::preview_dispose,
+            preview::player_pause_acked,
         ])
         .run({
             #[allow(unused_mut)]