feat(hwdec): add Android MediaCodec backend

[justin13888]

Closes #30

Summary

• add the Android API 29+ hardware still decoder using NDK AMediaCodec with an AImageReader CPU-readable output surface
• use JNI MediaCodecList only for the definitive API-29 hardware and alias classification, then open each eligible component by exact name through AMediaCodec
• support HEVC Main/Main10 and AV1 Profile 0, normalize output to dense I420/P010, reject silent 10-bit downconversion, and retry all eligible hardware components within one five-second deadline
• expose explicit, retryable, same-VM-idempotent JavaVM initialization through rawshift-hwdec, rawshift-image, and the facade
• add reproducible lossless fixtures, host-side parser/layout tests, Android cross-target CI, and an ARM64 Gradle physical-device harness with cold/warm session-reuse reporting

Android API boundary

MediaCodec is the sole Android decoding backend. Media3 and ExoPlayer orchestrate playback over MediaCodec, while ImageDecoder consumes complete image sources and is not the HEIF/AVIF item-codestream boundary needed here. Java MediaCodecList is used only because MediaCodecInfo.isHardwareAccelerated and isAlias are the public definitive classification methods starting at API 29; decoding and output acquisition remain NDK AMediaCodec and AImageReader.

Primary contracts:

• https://developer.android.com/reference/android/media/MediaCodecList
• https://developer.android.com/reference/android/media/MediaCodecInfo
• https://developer.android.com/ndk/reference/group/media
• https://developer.android.com/reference/android/media/ImageReader
• https://developer.android.com/reference/android/media/MediaCodecInfo.CodecCapabilities#COLOR_FormatYUVP010

Validation completed

• RUSTFLAGS=-Dwarnings cargo test --workspace
• RUSTFLAGS=-Dwarnings cargo clippy --workspace --all-targets
• Android ARM64 Clippy for rawshift-hwdec with mediacodec
• Android ARM64 checks for rawshift-image and the rawshift facade with hw-mediacodec
• Android ARM64 Clippy for the standalone JNI harness crate
• Gradle 9.4.1 help with all warnings enabled
• Debug and release JNI merge dry-runs verify buildRust is automatically wired as a generated-source dependency
• Gradle wrapper SHA-256 verified; shell scripts syntax-checked

Physical-device merge gate — pending

This remains draft until both report blocks are attached:

• API 29 ARM64 physical device with hardware HEVC and AV1: byte-exact 8-bit HEVC/AV1, cold and warm
• API 33+ ARM64 physical device with hardware HEVC and AV1: the same 8-bit checks plus byte-exact P010 from at least one codec

The instrumentation test rejects emulators because their codec inventory and acceleration path may depend on host passthrough. No performance threshold is imposed; cold_us and warm_us are diagnostic. Run instructions and pinned Linux/macOS build toolchains are in android/mediacodec-harness/README.md.

The current development host has no Android SDK/NDK, adb, or attached device, so no device report is claimed in this PR yet.