Phase 4 (partial) — stream value types + InnerTube /player helpers

Lands the data shapes + the HTTP layer for stream extraction. The
extractor orchestrator + DASH manifest creator are deferred to the
next session — the parsing logic is dense enough to want a focused
pass.

src/stream/
  * mod.rs       — StreamInfo + StreamInfoItem (full + 'card' shapes)
                   mirroring NPE StreamInfo.java + StreamInfoItem.java
  * delivery.rs  — DeliveryMethod (Progressive/Dash/Hls/Torrent)
  * audio.rs     — AudioStream (itag, format, url, bitrate, codec,
                   audio_track_id, content_length, etc.)
  * video.rs     — VideoStream (itag, format, url, resolution, fps,
                   bandwidth, codec, video_only flag)
  * subtitles.rs — SubtitlesStream (url, lang, auto_generated, mime)

src/youtube/stream_helper.rs
  * generate_content_playback_nonce() — 16-char LCG-shuffled cpn
  * get_web_metadata_player_response       (microformat + thumbnails only)
  * get_web_embedded_player_response       (embed-url + signatureTimestamp)
  * get_android_player_response            (full Android /player + poToken)
  * get_android_reel_player_response       (no-poToken fallback)
  * get_ios_player_response                (iOS — flagged with 917 KiB cap
                                            warning in the doc comment)

Per-helper headers + URL shapes match audit Track C §2.7 verbatim:
Android/iOS hit gapis endpoint with mobile UA; WEB family hits
www.youtube.com with the WEB headers.

Tests: 64 lib unit pass (up from 62 in Phase 3).

Next session: full stream_extractor.rs orchestrator + dash_manifest/
creator + Phase 4 done-when smoke (extract NCS Spektrem).

src/lib.rs

@@ -13,6 +13,7 @@ pub mod metainfo;
 pub mod newpipe;
 pub mod page;
 pub mod service;
+pub mod stream;
 pub mod youtube;
 
 pub use downloader::{Downloader, Request, Response};