feat: POC — bark (Ark) as an alternative spending backend [DO NOT MERGE]

[jvsena42]

Warning

This is a proof of concept and must not be merged. It exists to evaluate Second's Bark (Ark) as a
replacement for the local ldk-node Lightning node. Ark funds created by this branch are not backed
up — the recovery phrase alone cannot restore them — and several flows are deliberately unfinished.
Use signet and trivial amounts only.

Description

Adds Bark (Second's Ark implementation) as an alternative spending backend, switchable at runtime from
Settings → Advanced. Savings is untouched: ldk-node keeps running and keeps owning the on-chain wallet, so
only the Lightning/spending half is swapped out.

Second publishes an official Kotlin/UniFFI binding, so this is an integration rather than a bindings project.

• Dependency & flavor — tech.second.bark:bark-android:0.12.2+bark-0.4.0 from Second's public GitLab
  Maven, plus a new signet flavor (the only test network with a Second-hosted Ark server). Signet reuses
  the testnet applicationId so the checked-in google-services.json resolves without a signet Firebase
  client; the trade-off is that signet and tnet cannot be installed side by side.
• BarkService — thin wrapper over uniffi.bark.Wallet, shaped after LightningService, with all rust
  calls on a dedicated ServiceQueue.ARK thread.
• BarkRepo — BarkState StateFlow plus an executeWhenRunning gate mirroring
  LightningRepo.executeWhenNodeRunning. uniffi.bark types are wrapped so they never reach UI state.
• Balances — DeriveBalanceStateUseCase branches on the selected backend, so every existing balance
  surface (home tiles, spending screen, send flows) works with no UI change.
• Activities — bark Movements are mapped to bitkit-core LightningActivity and inserted through the
  existing path, so the activity list, detail screen, tags and contacts all work unchanged.
• Transfers — offboard (spending → savings) is one step. Board (savings → spending) is two, because bark
  boards from its own on-chain wallet; a persisted state machine survives the app being killed between them.
• VTXO maintenance — a 6h WorkManager worker plus a foreground pass run maintenanceDelegated(), the
  mobile refresh path. This is not optional: VTXOs expire (~28 days, ~3 days for Lightning receives) and the
  server can sweep them.
• Switch gate — the toggle is blocked unless the spending balance is 0, no channels are open, no transfer
  is in flight and no Ark exit is in progress. When blocked on a balance it deep-links into the existing
  Transfer → Savings flow.

Two findings worth calling out

1. bark cannot reuse bitkit's on-chain wallet. bark exposes CustomOnchainWalletCallbacks, which would
   have given one-step boarding and a single UTXO set — but it needs prepareTx → PSBT and finishPsbt, and
   ldk-node's OnchainPaymentInterface (verified against 0.7.0-rc.62 sources) has no PSBT API at all. bark
   therefore keeps a second on-chain wallet on the same seed. It is never presented as "Savings".
2. Both build risks flagged during planning are resolved. libbark_ffi_kotlin.so packages cleanly
   without bark's README-mandated useLegacyPackaging = true (which would have changed packaging for
   ldk-node/bitkit-core/paykit/vss too), and the arm64 lib is 16 KB page-size compliant (PT_LOAD align
   0x4000).

Deliberately out of scope

• No backup of the bark database. Mandatory before this is anything more than a POC.
• Send/receive UI routing is wired at the repo layer; the send/receive ViewModels still need to branch on
  the backend, and the Transfer screens still need bark-specific copy.
• Blocktank/CJIT entry points are not yet hidden in bark mode (the Advanced-tab Lightning rows are).
• WakeNodeWorker (FCM push wake) is untouched — bark has no push, so waking on an LSP notification would be
  a no-op. Background liveness comes from BarkMaintenanceWorker instead.

Preview

QA Notes

Journeys live in journeys/bark/ and are written for the android-cli journey runner:

• bark-toggle-gate.xml — the switch is blocked with a spending balance and offers Transfer To Savings
• bark-enable.xml — the no-backup warning shows, and Lightning-only rows disappear once enabled
• bark-receive-lightning.xml — BOLT11 receive over Ark lands in balance and activity
• bark-send-lightning.xml — BOLT11 send decreases balance and records a sent activity
• bark-board.xml — savings → spending shows as in-transfer until the board completes
• ldk-regression.xml — run this on a dev build; guards that the shipping ldk-node path is untouched
  and that the Ark toggle is hidden on networks with no Ark server

Unit tests: BarkMovementsTest.kt (movement → activity mapping), CanSwitchSpendingBackendUseCaseTest.kt
(every allow/block branch, both directions), BarkTransferRepoTest.kt (board state machine, including
resume-after-death and failure keeping the pending board), plus ContentViewTest.kt for the extracted
transferSavingsStartRoute helper.

Validation run: just compile, just test, just lint all pass, and assembleSignetDebug produces an APK
with libbark_ffi_kotlin.so present. compileMainnetDebugKotlin needs a local
app/src/mainnet/google-services.json, which is gitignored and not present in a fresh worktree.

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