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/Cargo.toml

@@ -44,6 +44,13 @@ android_logger = { version = "0.14", default-features = false }
 reqwest = { version = "0.12", default-features = false, features = ["rustls-tls", "gzip", "stream"] }
 quick-xml = "0.36"
 futures = "0.3"
+# RYD + SponsorBlock JSON clients (net.rs). serde/serde_json are already in
+# the dependency tree via strawcore-core; declaring them here lets the
+# wrapper parse the two small enrichment endpoints directly. sha2 powers
+# SponsorBlock's privacy-preserving SHA-256 hash-prefix lookup.
+serde = { version = "1", features = ["derive"] }
+serde_json = "1"
+sha2 = "0.10"
 
 [build-dependencies]
 uniffi = { version = "0.28", features = ["build"] }