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.

Cargo.lock

@@ -1750,6 +1750,12 @@ version = "1.10.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "6dbf3de79e51f3d586ab4cb9d5c3e2c14aa28ed23d180cf89b4df0454a69cc87"
 
+[[package]]
+name = "httpdate"
+version = "1.0.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "df3b46402a9d5adb4c86a0cf463f42e19994e3ee891101b1841f30a545cb49a9"
+
 [[package]]
 name = "hyper"
 version = "1.10.1"
@@ -2398,6 +2404,7 @@ name = "metadata"
 version = "0.1.0"
 dependencies = [
  "futures",
+ "httpdate",
  "image",
  "log",
  "reqwest 0.12.28",