vc=85: image caching + SB/RYD clients -> Rust + crash/autoplay fixes

- Thumbnails + channel icons stay cached: pin an explicit 256MB Coil disk
  cache + sized memory cache via SingletonImageLoader.Factory. Coil's
  default disk cap is 2% of the device's free space, so on a storage-tight
  phone the subs feed (most image-heavy screen) thrashed it and
  re-downloaded thumbnails on every visit.
- SponsorBlock + Return-YouTube-Dislike clients moved Kotlin -> Rust
  (strawcore net.rs: fetchSponsorSegments / fetchRydVotes). SponsorBlock
  keeps its privacy-preserving SHA-256 hash-prefix lookup. Kotlin is now a
  thin shim mapping the FFI records onto the SbSegment/RydVotes domain
  types; behavior identical. Migration #2 of "all backend -> Rust".
- Fix crash: extract_channel_id sliced the channel URL by a length derived
  from a lowercased copy of itself; to_lowercase() can change byte length
  on non-ASCII, so a non-ASCII URL tail could panic across the FFI and
  abort the app on a feed refresh. Now matches the prefix case-insensitively
  against the original with length + char-boundary guards.
- Fix autoplay hijack: advancing to the next video resolves over ~500ms; if
  you manually start a different video meanwhile, autoplay would replace
  your choice with the stale next-up. Added a staleness fence.

Verified: cargo check/test/clippy on the wrapper, full Android
compileDebugKotlin green, adversarial FFI pre-push audit passed.

rust/strawcore/src/lib.rs

@@ -13,6 +13,7 @@ use std::sync::Once;
 mod channel;
 mod error;
 mod feed;
+mod net;
 mod runtime;
 mod search;
 mod stream;
@@ -20,6 +21,7 @@ mod stream;
 // Re-exports so UniFFI sees the types at the crate root for macro discovery.
 pub use channel::ChannelInfo;
 pub use error::StrawcoreError;
+pub use net::{RydVotes, SponsorSegment};
 pub use search::{Page, SearchItem};
 pub use stream::{AudioStreamItem, ResolvedStreams, StreamInfo, VideoStreamItem};