[#1924] Add Android shielded-voting JNI foundation, round lifecycle, and delegation prep + signing

[greg0x]

Part of zodl-inc/zodl-android#2193.
Slice of zcash#1924.

Why

Land the Android-side JNI surface needed before delegation proof generation. Pulls the Rust backend onto the released zcash_voting dependency, adds the voting DB handle and round lifecycle, and exposes the prep + signing surface: bundle setup, hotkey derivation, governance PCZT construction, sighash extraction, and spend-auth signature extraction.

Vertical slice — every new JNI symbol gets a matching Rust handler, Kotlin external fun, FFI model, and typesafe wrapper. Surface is entirely internal; no public Synchronizer API yet.

What's in

Dependencies — orchard hard-pinned to =0.13.1 with unstable-voting-circuits; zcash_voting 0.5.3 with default-features = false (no PIR / tree-sync / networking pulled in).

JNI (backend-lib/src/main/rust/voting/)

Module	Exports
db.rs	open / close voting DB
rounds.rs	initRound, getRoundState, listRounds, getBundleCount, getVotes, clearRound, deleteSkippedBundles
notes.rs	computeBundleSetup, setupBundles, generateHotkey
delegation.rs	buildGovernancePcztJson, extractPcztSighash, extractSpendAuthSig
share_tracking.rs	computeShareNullifier
util.rs	warmProvingCaches
helpers.rs	byte-boundary validators, width-checked jint/jlong casts, network/coin-type/branch-id, Orchard FVK + hotkey raw-address derivation, FFI builders
json.rs	JsonNoteInfo, trimmed JsonGovernancePczt envelope, hex enc/dec, json_from_jstring

Kotlin (sdk-lib/.../internal/)

• jni/VotingRustBackend.kt — external fun declarations; nested VotingDb handle guards every native call behind a Mutex and dispatches on SdkDispatchers.DATABASE_IO.
• TypesafeVotingBackend.kt + Impl — suspend wrappers, native { … } helper that calls RustBackend.loadLibrary() before every dispatched block (idempotent; matches RustDerivationTool / RustEip681Tool / TorClient), JSON → GovernancePcztResult parsing.
• model/voting/FfiVotingModels.kt — FfiRoundState, FfiRoundSummary, FfiBundleSetupResult, FfiVotingHotkey, HotkeySecretKey, HotkeyPublicKey (all with size-validated constructors).
• jni/JniConstants.kt — hotkey key size constants.
• ext/BlockExt.kt — String.fromHex() made strict (rejects odd length and non-hex characters) since it's now on the JNI parse path.

Not in scope

• PIR precompute
• Delegation proof generation
• Delegation submission (Keystone or otherwise)
• Vote tree sync, VAN witnesses, vote commitment
• Recovery / share-tracking state beyond the linkage symbol
• Public Synchronizer voting API

Review focus

• Dependency pin rationale — orchard hard-pin and default-features = false on zcash_voting are deliberate; see comment block in backend-lib/Cargo.toml.
• Governance PCZT JSON envelope — only pczt_bytes / rk / action_index / pczt_sighash cross JNI. The rest of the delegation-data fields (alpha, rseed_*, padded_note_secrets, gov_nullifiers, …) are persisted by zcash_voting's store_delegation_data and read back in the proof-generation slice.
• Byte-boundary validation — java_bytes, java_bytes_at_least, java_bytes32, fixed-width require_len / require_min_len, and u32_to_jint / u64_to_jlong / usize_to_jint cast helpers.
• setupBundles parity check — chunker output is recomputed in Rust and compared against db.setup_bundles; on mismatch the JNI returns Err. The DB write has already happened at that point, so callers should treat the mismatch as a bug rather than a recoverable error.
• Hotkey derivation — hotkey_orchard_raw_address_from_wallet_seed derives the Orchard raw address from the wallet seed at the supplied account_index, using Scope::External at index 0.
• VotingDb handle — Mutex-guarded native calls, close() nulls the handle, withHandle errors on use-after-close.

Validation

• cargo check --manifest-path backend-lib/Cargo.toml --locked
• cargo fmt --manifest-path backend-lib/Cargo.toml --check
• ./gradlew :backend-lib:compileReleaseKotlin :sdk-lib:compileReleaseKotlin
• ./gradlew ktlint detektAll
• ./gradlew checkProperties
• git diff --check