[ovitrif]

Dependency:

• feat: pay contacts via spark #184
• feat: arkade spending in bitkit-core #183

On hold: Bitkit goes with Spark for Lightning for now, so this stays parked as it is.

This PR lets an Arkade offboard pay a Paykit contact who only accepts on-chain bitcoin, as a service leg in the Paykit router.

Description

• Merges the Arkade wallet of feat: arkade spending in bitkit-core #183 into the service routes of feat: pay contacts via spark #184, so one bitkit-core build carries both.
• Adds arkade_paykit_route_leg(endpoint, amount_sat): a leg that offboards exactly amount_sat to the contact's on-chain address, with the operator's exit fee on top, or None while the available Arkade balance cannot cover both. The fee comes from the operator's own fee programs, evaluated the way arkade_offboard evaluates them. The app passes the leg to pubky_swaps_route_paykit_payment in service_legs.
• Adds arkade_offboard_exact(address, amount_sat), which runs the leg: the contact receives exactly amount_sat in the next batch, and the exit fee comes on top. arkade_offboard keeps its behaviour, where the fee comes out of the amount.
• An Arkade leg reports operator atomicity: the operator co-signs the batch that pays the contact.
• Cut as the prerelease 0.8.0-routes.1, which the Arkade app branches use.

Cross-platform impact

• New exports: arkade_paykit_route_leg and arkade_offboard_exact. No existing export changes beyond feat: pay contacts via spark #184's.

What it needs to ship

• feat: pubky swaps as paykit route #182, feat: arkade spending in bitkit-core #183 and feat: pay contacts via spark #184 merged first; this PR then reduces to the two exports.
• A device journey against a local arkade-regtest stack, with the app killed mid-batch and a seed restore while the offboard is pending as first-class cases.

Preview

N/A — library change; no recording yet.

QA Notes

Setup

N/A — no device testing required: the change is covered by unit tests, and no app journey exists yet.

Automated Checks

• ran cargo fmt --check, cargo clippy --lib --tests (no findings in arkade or pubky_swaps), cargo test --lib pubky_swaps and cargo test --lib modules::arkade: pass
• not run: Arkade's regtest tests, which need an arkade-regtest stack
• no unit test covers arkade_paykit_route_leg itself: it needs a running wallet and operator

Models used

• Planning/scoping: claude-opus-5-5 (reasoning: medium)
• Implementation: claude-opus-5-5 (reasoning: medium)
• Review: Not performed

[ovitrif]

Closing this PR: after a full mainnet QA run on Android and iOS, Bitkit goes with Spark for Lightning.

Arkade worked end to end with real funds (LN send and receive, Lightning address, transfers to and from Savings), but it needs too much built and run on our side: chain access through block explorers, our own LN swap client and the activity history. The branch stays for reference.