Draft: Add Android coinholder polling backend

[greg0x]

Summary

• add the Android SDK voting backend for coinholder polling
• expose voting recovery, Keystone, and share-tracking primitives needed by the app flow
• harden Android voting state transitions for round prep, resume, and multi-proposal submission parity

Issues

• MOB-1109 Integrate and expose v1 implementation of shielded voting
• implement Coinholder Polling zodl-inc/zodl-android#2193

Companion PRs

• companion app PR: zodl-inc/zodl-android#2206

Recording

• https://cleanshot.com/share/rqGnp2Mg

Notes

• this branch includes the original MOB-1108 SDK backend work plus the additional parity and recovery fixes needed to get live Android voting through end-to-end testing

Testing

• ./gradlew :sdk-lib:assembleDebug

[greg0x]

We are splitting this up into multiple PRs but keeping this branch + draft PR as the integration reference target

[greg0x]

This is stale now

#1953 is the last slice from the PR splits

[greg0x]

Handoff context: updating the shielded-vote known state onto upstream main.

Goal

• Preserve the known shielded-vote state, including fixes committed while the SDK split PRs were under review.
• Move that state onto upstream main, which already contains PR6/PR7 split work (SDK #1951 and SDK #1952) plus SDK #1937.
• Make sure we do not lose parallel shielded-vote fixes, but also do not resurrect code that was intentionally removed during the split PR reviews.
• PR8 (SDK #1953, greg/voting-recovery-share-tracking) is a separate review/test track. It was used only as a reference where it overlapped with recovery/JSON conflicts; this was not a wholesale PR8 merge.

Current state

• This PR is valargroup:shielded-vote -> main; the local branch head used for this handoff is 2462e64d.
• The remaining SDK delta over main is now focused on voting JNI/Rust recovery and integration APIs: backend-lib/Cargo*, VotingRustBackend, JniVotingModels, Rust voting/*, and internal TypesafeVotingBackend*.
• The zodl companion PR for exercising this SDK branch is valargroup/zodl-android#24.

What changed during the rebase

• Accepted upstream main as canonical for the landed PR6/PR7 split behavior from SDK #1951 and SDK #1952.
• Accepted upstream main as canonical for SDK #1937 broadcaster behavior, even though SDK #1937 was not originally part of shielded-vote.
• Reapplied only the remaining shielded-vote integration/fix surface on top of that base: voting DB/JNI bridge, recovery state APIs, commitment bundle storage, share delegation tracking, and SDK internal wrappers.
• Kept the hotkey-seed direction where voting proof/signing code uses the explicit hotkey seed rather than deriving the hotkey address from the wallet seed plus address index.
• Kept the recovery lookup behavior that treats missing recovery rows as expected missing state instead of crashing the caller.
• Kept the unpaired governance PCZT retry handling, then dropped the paired-governance retry workaround that was no longer needed after the reviewed split changes.

Decisions made

• Do not reintroduce code removed by the landed split PRs. In particular, the old blake2b dependency path and old json.rs module/path stayed dropped.
• Keep SDK #1937 broadcaster changes from main; do not let older branch diffs remove or revert those files.
• Keep recovery/share-tracking fixes that were already carried by shielded-vote where they were actual fixes.
• Treat PR8 as separate. If a conflict was specifically recovery/JSON-related, the PR8 version was useful context; otherwise PR8 stayed out of this branch.

PR8 overlap kept in this branch

These are best understood as shielded-vote fixes that overlap with PR8, not as a PR8 merge:

• Native recovery storage surface: delegation/vote tx hash storage and lookup, commitment bundle storage and lookup, recovery state clearing, share delegation recording/listing, confirmation marking, and sent-server tracking.
• JNI/Kotlin models needed by that surface: JniCommitmentBundleRecord and JniShareDelegationRecord.
• Full vote commitment bundle JSON serialization/deserialization using serde / serde_json so stored commitments can be recovered and reused.
• zcash_voting bump to 0.6.0 as part of the rebased voting stack.
• Hotkey seed propagation into delegation/signing paths where it was required by the recovery/signing model.
• Missing-row handling for recovery lookups so expected absence is surfaced as missing state rather than an exception path.

PR8 work not brought over

• PR8 was not merged wholesale.
• PR8-only test expansions such as the larger TypesafeVotingBackendImplTest and JniVotingModelsTest changes were left with SDK #1953 rather than pulled into this rebase.
• PR8 old-base artifacts that conflicted with current main, especially broadcaster-file deletions/reversions relative to SDK #1937, were not carried over.
• Non-recovery/non-JSON PR8 review nits were kept out unless they were needed to make this rebased shielded-vote branch coherent.

Suggested verification path

• In this SDK repo, run ./scripts/ci-local.sh fast, then ./scripts/ci-local.sh quick. Because this touches the Kotlin/JNI/Rust voting boundary, ./scripts/ci-local.sh full is the right final local validation before treating it as ready.
• Then test through zodl using valargroup/zodl-android#24 with SDK_INCLUDED_BUILD_PATH pointing at this SDK checkout. In the current workspace, zodl uses SDK_INCLUDED_BUILD_PATH=../zcash-android-wallet-sdk.
• Focus manual/regression testing on voting snapshot/witness generation, witness storage, vote commitment generation, cast-vote signing, hotkey seed propagation, and recovery/share tracking.

[p0mvn]

Aligned with #1953