Phase 4 (complete) — stream_extractor orchestrator

Wire the Android-primary fetch path + JSON-walking + URL post-processing
into a single stream_info(video_id) entry point. Mirrors NPE
YoutubeStreamExtractor.onFetchPage() per audit Track C §1.2.

src/youtube/stream_extractor.rs
  * stream_info(video_id) + stream_info_with(video_id, options)
  * fetch_android — reel endpoint (anonymous) OR /player (with po_token)
  * check_playability_status — maps to ContentUnavailable variants
    (AgeRestricted, GeoRestricted, Paid, Private, YoutubeMusicPremium,
    AccountTerminated, Other)
  * is_player_response_not_valid — decoy-video detection
  * populate_video_details + populate_microformat + populate_streams +
    populate_manifests + populate_captions
  * process_url — sig deobf path (signatureCipher → JS function call)
    + unconditional nsig deobf + cpn append + pot append
  * build_video_progressive / build_video_only / build_audio +
    push_*_dedup helpers (FIX: NPE bug — dedup by itag id, not by
    mediaFormat.id which collides 140/141)

Consolidated stream_helper's local ExtractionError into the crate-wide
exceptions::ExtractionError with a new DownloaderMissing variant.

Tests: 73 lib unit pass (+9 since Phase 3) + 7 new Phase 4 offline
integration tests = 80 lib green. Live YT end-to-end smoke deferred
to Straw integration; the code path is in place.

src/youtube/mod.rs

@@ -8,5 +8,6 @@ pub mod constants;
 pub mod itag;
 pub mod js;
 pub mod parsing;
+pub mod stream_extractor;
 pub mod stream_helper;