Expose asset-lock drain (send-all) through the identity top-up FFI, and return the L1 funding txid

[HashEngineering]

Problem. IdentityCredits.topUpFromCore / topUpIdentityFromCore is exact-amount only, so a wallet cannot offer "Max" (empty-wallet) identity top-ups through the SDK. The Rust builder work already exists — rust-dashcore #915 (9c87ccb3) added drain mode to the asset-lock builders — but it is unreachable from the SDK:

1. The platform workspace pins rust-dashcore rev 70d4bf8e (root Cargo.toml:55-62), which is 5 commits older than 9c87ccb3 and predates the feature entirely (git merge-base --is-ancestor verified).
2. Even at rust-dashcore tip, the single key-wallet FFI entry point serving registration AND top-up — wallet_build_and_sign_asset_lock_transaction (key-wallet-ffi/src/transaction.rs:770) — deliberately pins the old behavior: it passes AssetLockFundingAccount::Bip44 { account_index } and drain = false (transaction.rs:841-849; the feat(rs-drive-abci)!: commit and vote extensions signature verification #915 commit message says so explicitly).

Ask.

• Bump the platform rust-dashcore pin past 9c87ccb3.
• Extend the key-wallet FFI (new entry point or versioned signature) with drain: bool and the funding-account family selector, honoring feat(rs-drive-abci)!: commit and vote extensions signature verification #915's invariants (drain requires exactly one credit output; fee deducted from the locked amount; CoinJoin funding is drain-only).
• Plumb through rs-platform-wallet-ffi → topUpIdentityFromCore → Kotlin IdentityCredits.topUpFromCore (and the Swift equivalent) as e.g. sendAll: Boolean (drain ⇒ amountDuffs ignored/0).

While touching this FFI: return the L1 funding txid. topUpIdentityFromCore returns only the post-transition credit balance. The host wallet needs the asset-lock txid to show the transaction (result screen, tx list) — today it must diff the asset_locks persistence table around the call, which is heuristic under concurrency. A (balance, outpoint) return (or out-param) removes that. Same applies to the registration/invite variants for consistency.

Wallet impact. dash-wallet gates Max top-ups off on the SDK route until this lands (SdkTopUpService KDoc references this issue); exact-amount top-ups are unaffected.

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Context: dashpay/dash-wallet#1520 (SDK cutover Phase 1B, asset-lock sends) / MO-998. Findings verified against integration/kotlin-sdk-stack + rust-dashcore dev @ 9c87ccb3.