[#2193] Add Android voting JNI round lifecycle

[greg0x]

Part of zodl-inc/zodl-android#2193.
Slice of zcash#1924 (umbrella draft PR being split into reviewable pieces).
Depends on zcash#1934 (first slice — zcash_voting dependency foundation).

Why

Second slice of the Android shielded-voting integration. zcash#1934 wires the zcash_voting crate into backend-lib; this PR adds the voting DB handle and round lifecycle so the round-keyed surface (init / state / list / get-votes / clear / delete-skipped) compiles end-to-end from Rust through JNI to Kotlin.

Foundation for the Coinholder Polling flow tracked in zodl-inc/zodl-android#2193 — no user-visible behavior on its own.

What's in

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

• db.rs — open/close native handle, setWalletId
• rounds.rs — initRound, getRoundState, listRoundsJson, getVotesJson, clearRound, deleteSkippedBundles
• share_tracking.rs — existing computeShareNullifier, lifted out of the monolithic module
• helpers.rs — shared JNI helpers: fixed-width byte validation (PROTOCOL_FIELD_BYTES = 32), jint/jlong widening, RoundState → FfiRoundState construction
• json.rs — round summary / vote record serialization, RoundPhase ↔ u32 mapping
• voting.rs — reduced to module roots + shared imports

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

• jni/VotingRustBackend.kt — external declarations for the new surface
• TypesafeVotingBackend.kt + impl — suspend wrappers, IO dispatcher, error checks, JSON → VoteRecord parsing
• model/voting/FfiVotingModels.kt — FfiRoundState, FfiRoundPhase

Not included

• hotkey generation
• bundle setup/count
• proving cache warmup
• delegation proving/submission
• vote commitment or share payloads
• recovery/share tracking
• public Synchronizer voting API

Review focus

Primary review track: core-dev. Please look at:

• native handle lifecycle (Box::into_raw / Box::from_raw across openVotingDb/closeVotingDb, Arc<VotingDb> usage)
• JNI boundary error semantics (panic-safe via catch_unwind, exception conversion)
• byte-array validation (java_bytes_exact on 32-byte protocol fields)
• RoundState / RoundSummary JSON and FFI mapping
• Kotlin surface is intentionally internal and minimal — just enough to compile and exercise the JNI

Swift counterparts:

• [#1661] Add voting FFI foundation helpers zcash/zcash-swift-wallet-sdk#1716
• [#1661] Add voting round recovery FFI zcash/zcash-swift-wallet-sdk#1722
• [#1661] Voting FFI in VotingRustBackend.swift (database, PR, VAN witness gen) zcash/zcash-swift-wallet-sdk#1719

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

Author

• Self-review your own code in GitHub web interface
• Add automated tests as appropriate
• Update the manual tests as appropriate
• Check the code coverage report for the automated tests
• Update documentation as appropriate
• Run the demo app and try the changes
• Pull in the latest changes from the main branch and squash your commits before assigning a reviewer

Reviewer

• Check the code with the Code Review Guidelines checklist
• Perform an ad hoc review
• Review the automated tests
• Review the manual tests
• Review the documentation as appropriate
• Run the demo app and try the changes

[greg0x]

zcash#1938