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/Cargo.lock

@@ -224,6 +224,15 @@ version = "2.13.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "b4388bee8683e3d04af747c73422af53102d2bd24d9eadb6cbc100baef4b43f8"
 
+[[package]]
+name = "block-buffer"
+version = "0.10.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3078c7629b62d3f0439517fa394996acacc5cbc91c5a20d8c658e77abd503a71"
+dependencies = [
+ "generic-array",
+]
+
 [[package]]
 name = "bumpalo"
 version = "3.20.3"
@@ -373,6 +382,15 @@ version = "0.4.32"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "cc14f565cf027a105f7a44ccf9e5b424348421a1d8952a8fc9d499d313107789"
 
+[[package]]
+name = "cpufeatures"
+version = "0.2.17"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "59ed5838eebb26a2bb2e58f6d5b5316989ae9d08bab10e0e6d103e656d1b0280"
+dependencies = [
+ "libc",
+]
+
 [[package]]
 name = "crc32fast"
 version = "1.5.0"
@@ -382,6 +400,26 @@ dependencies = [
  "cfg-if",
 ]
 
+[[package]]
+name = "crypto-common"
+version = "0.1.7"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "78c8292055d1c1df0cce5d180393dc8cce0abec0a7102adb6c7b1eef6016d60a"
+dependencies = [
+ "generic-array",
+ "typenum",
+]
+
+[[package]]
+name = "digest"
+version = "0.10.7"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9ed9a281f7bc9b7576e61468ba615a66a5c8cfdff42420a70aa82701a3b1e292"
+dependencies = [
+ "block-buffer",
+ "crypto-common",
+]
+
 [[package]]
 name = "displaydoc"
 version = "0.2.6"
@@ -542,6 +580,16 @@ dependencies = [
  "slab",
 ]
 
+[[package]]
+name = "generic-array"
+version = "0.14.7"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "85649ca51fd72272d7821adaf274ad91c288277713d9c18820d8499a7ff69e9a"
+dependencies = [
+ "typenum",
+ "version_check",
+]
+
 [[package]]
 name = "getrandom"
 version = "0.2.17"
@@ -1425,6 +1473,17 @@ dependencies = [
  "serde",
 ]
 
+[[package]]
+name = "sha2"
+version = "0.10.9"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a7507d819769d01a365ab707794a4084392c824f54a7a6a7862f8c3d0892b283"
+dependencies = [
+ "cfg-if",
+ "cpufeatures",
+ "digest",
+]
+
 [[package]]
 name = "shlex"
 version = "1.3.0"
@@ -1499,6 +1558,9 @@ dependencies = [
  "quick-xml",
  "reqwest",
  "rquickjs-sys",
+ "serde",
+ "serde_json",
+ "sha2",
  "strawcore-core",
  "thiserror 1.0.69",
  "tokio",
@@ -1771,6 +1833,12 @@ version = "0.2.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "e421abadd41a4225275504ea4d6566923418b7f05506fbc9c0fe86ba7396114b"
 
+[[package]]
+name = "typenum"
+version = "1.20.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b6f5e870be6c3b371b77fe0ee0bafb859fa4964b4404c27de1d380043c4dda20"
+
 [[package]]
 name = "unicase"
 version = "2.9.0"
@@ -1950,6 +2018,12 @@ version = "0.2.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "06abde3611657adf66d383f00b093d7faecc7fa57071cce2578660c9f1010821"
 
+[[package]]
+name = "version_check"
+version = "0.9.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0b928f33d975fc6ad9f86c8f283853ad26bdd5b10b7f1542aa2fa15e2289105a"
+
 [[package]]
 name = "want"
 version = "0.3.1"