feat(kotlin-sdk): transaction decoder JNI binding over key-wallet-ffi transaction_decode

[bfoss765]

Adds the Android/Kotlin counterpart of the Swift TransactionDecoder (#3981): a single JNI export (TxDecodeNative.decodeTransaction) over key-wallet-ffi's transaction_decode — already present in v4.1-dev's rust-dashcore pin via upstream rust-dashcore#870, so no vendored Rust changes are needed. The JNI layer marshals the decode result into a packed big-endian BLOB parsed by the new org.dashfoundation.dashsdk.keywallet.TransactionDecoder into DecodedTransaction (txid + per-input prev outpoints / unauthenticated P2PKH sender hints + per-output address/value/script).

The JNI-free decode_to_blob core is host-tested (7 Rust tests), including a fixture blob pinned byte-for-byte against Kotlin's TransactionDecoderTest (4 tests) as a cross-language layout pin.

Verified: cargo test -p rs-unified-sdk-jni (17 tests) and :sdk:assembleRelease + sdk unit tests all pass against the pin's transaction_decode signature (no API drift).

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📥 Commits

Reviewing files that changed from the base of the PR and between 8b466ab and 40aa94f.

📒 Files selected for processing (5)

• packages/kotlin-sdk/sdk/src/main/kotlin/org/dashfoundation/dashsdk/ffi/TxDecodeNative.kt
• packages/kotlin-sdk/sdk/src/main/kotlin/org/dashfoundation/dashsdk/keywallet/TransactionDecoder.kt
• packages/kotlin-sdk/sdk/src/test/kotlin/org/dashfoundation/dashsdk/keywallet/TransactionDecoderTest.kt
• packages/rs-unified-sdk-jni/src/lib.rs
• packages/rs-unified-sdk-jni/src/tx_decode.rs

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[thepastaclaw]

✅ Final review complete — no blockers (commit 40aa94f)

[shumkov]

Reviewed against v4.1-dev with focus on FFI fidelity and Swift parity. This is a faithful counterpart of the Swift TransactionDecoder (#3981): same key-wallet-ffi transaction_decode at the same rust-dashcore pin, exact field parity, and the cross-pinned fixture blob (Rust + Kotlin) is a nice touch. Mergeable as-is. Minor, non-blocking:

• rs-unified-sdk-jni/src/tx_decode.rs:110-116 — script encoding is half-defensive: length is always written but bytes only if ptr non-null; make the null branch write length 0 so the blob can't shift.
• net_from_ord is now the third private copy (also transactions.rs, identity.rs) — worth hoisting into support.rs.
• Follow-up idea (upstream, not this PR): transaction_decode exposes no version/locktime/special-tx payloads, so neither platform can render ProRegTx/AssetLock details — a key-wallet-ffi enhancement both SDKs would pick up together.

[shumkov]

Post-merge follow-up only; I found no blocker in the decoder architecture.

Kotlin exposes prevVout as signed Int, so the unsigned coinbase sentinel 0xFFFFFFFF appears as -1, while the Swift wrapper uses UInt32. Consider exposing UInt/Long, or explicitly document that Kotlin preserves the raw u32 bits in a signed value. The current test intentionally pins -1, so this is a host-parity/API clarity issue rather than a decoding bug.

[bfoss765]

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[coderabbitai]

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[bfoss765]

Thanks for the review and approval. All three notes are implemented in a small follow-up PR (opening shortly): the script-encoder null branch now writes length 0 (plus the same guard one level up for the input/output arrays, with lying-fixture tests), net_from_ord is hoisted into support.rs (we found and removed a fourth copy in persistence.rs), and prevVout now documents the raw-u32-bits contract with a prevVoutUnsigned convenience accessor — the pinned -1 test is untouched.

Separately: could you hit "Re-run failed jobs" on the x86_64 emulator check on #4182 and #4183? As you noted, the failure predates the diffs — it's the known emulator-boot flake, and we don't have rerun rights.