[rdlabo]

Native snapshot handoffs repeatedly encoded PNGs and decoded unchanged images. This PR adds a packaged Rust PNG encoder, bounded two-worker postprocessing, and reuse of eligible captures/files/decoded images. Consumers do not need Rust or an NDK.

Performance

Total time includes capture readiness, capture, copies/FFI, PNG encoding, file transport, Web image decode and drawing confirmation. Each comparison interleaved modes, excluded three warmup rounds, and measured 30 fresh captures per mode on a 12-control screen.

Platform encoder → serial Rust encoder

Environment	Before p50 / p90	After p50 / p90	p50 reduction
iOS 27 simulator, PNG16 Display P3	454.07 / 493.98 ms	352.21 / 388.27 ms	22.4%
Android 15 emulator, 720p	721.75 / 1,035.36 ms	599.02 / 779.72 ms	17.0%

Serial Rust → two-worker Rust (same codec/settings)

Environment	Before p50 / p90	After p50 / p90	p50 reduction
iOS simulator	355.09 / 394.38 ms	294.78 / 305.37 ms	17.0%
Android emulator	605.77 / 885.91 ms	555.29 / 871.56 ms	8.3%

These are separate experiments; their reductions must not be added. PNG size changes by +2.3% on iOS and −15.2% on Android in the encoder comparison, and is unchanged between worker modes. These fixture results are not physical-device or universal speedup claims; Android's final p90 concurrency improvement is small.

Behavior and correctness

• Keep shared Rust notices in one canonical resource directory consumed by SwiftPM/CocoaPods and Android; retain only NDK-specific notices separately. Package checks compare every bundled notice with its original.
• Preserve iOS 16-bit Display P3/ICC and alpha, with platform encoding fallbacks. Process large crops serially.
• Require explicit native reuse acknowledgement before Web reuses a decoded image. Repeated unchanged preparations and entry/return add no capture, encode, file write/read or decode in the integration test.
• Reject Android cache reuse when relative raster-crop placement or source draw order changes; retain reuse for a common translation and preserve token/file ownership.

Local validation: TypeScript/ESLint/format checks, native artifact verification, iOS image-reuse integration, and Android Kotlin formatting/Release lint passed. Four Android emulator regressions passed, including common translation, relative movement, draw-order changes and comparison against fresh-capture pixels.