feat(platform-wallet): reserve DashPay payout addresses without Core funding

[PastaPastaPasta]

Issue being fixed or feature implemented

DashPay contact payments currently expose a combined address-resolution and Core-funded send operation. Wallets with Platform or shielded funds cannot obtain a fresh DIP-15 contact destination without selecting transparent Core inputs.

What was done?

Add DashPayView::reserve_payment_address and the Swift ManagedPlatformWallet.reserveDashPayPaymentAddress(fromIdentityId:toContactIdentityId:) wrapper through the C FFI. The operation completes the seed-verified deferred contact-crypto drain, consumes an address from the authoritative wallet/owner/contact external account pool, and persists and flushes its used flag before returning it.

Core sends and external payout reservations share the derivation helper and a per-wallet gate that keeps their pool snapshots ordered. Core broadcasts release the gate while awaiting the network, then reacquire it before a definitive rejection's address rollback. The Swift wrapper retains its mnemonic resolver throughout the FFI call and frees the returned string.

Callers reserve only when submitting a confirmed payment. Exposed addresses stay consumed after cancellation, rejected withdrawal, or an unknown outcome; this API does not submit the withdrawal or record payment history. The companion iOS change owns the submitted-withdrawal history.

How Has This Been Tested?

• Rust DashPay payments unit suite: 61 passed, including zero-Core-balance reservations, wallet/identity/contact scoping, persisted rotation shared with a subsequent Core send, store/flush failure, wrong-seed rejection, deferred contact setup, and preservation across a concurrent Core rejection.
• cargo clippy -p platform-wallet -p platform-wallet-ffi --lib (only pre-existing upstream warnings).
• cargo fmt -p platform-wallet -p platform-wallet-ffi --check and git diff --check.
• iOS simulator release FFI build and SwiftExampleApp Xcode build.

Real testnet validation used two registered DashPay contacts: a 0.02 DASH Platform withdrawal with zero Transparent balance and a user-submitted 0.1 DASH Shielded withdrawal paid the reserved addresses. Recipient receipt verification used a combined test build with the separate AssetUnlock discovery fix in dashpay/rust-dashcore#1004. Screenshots and exact build provenance.

The receipt fix needs a separate upstream SDK dependency update. This PR only adds address reservation and leaves the existing rust-dashcore dependencies unchanged.

Breaking Changes

None. The existing Core-funded send API remains available.

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated relevant unit/integration/functional/e2e tests
• I have added "!" to the title and described breaking changes in the corresponding section if my code contains any
• I have made corresponding changes to the documentation if needed

For repository code-owners and collaborators only

• I have assigned this pull request to a milestone

This pull request was created by Codex.

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📥 Commits

Reviewing files that changed from the base of the PR and between ca1612e and d0bff14.

📒 Files selected for processing (4)

• packages/rs-platform-wallet-ffi/src/dashpay.rs
• packages/rs-platform-wallet/src/wallet/identity/network/payments.rs
• packages/rs-platform-wallet/src/wallet/persister.rs
• packages/swift-sdk/Sources/SwiftDashSDK/PlatformWallet/ManagedPlatformWallet.swift

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[PastaPastaPasta]

Superseded by #4623 so the Rust workspace and Swift SDK CI jobs run (they skip fork heads).

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🤖 Posted autonomously by Claude on behalf of pasta.