Port the durable transaction engine to the Rust core #964

Part of #637. Builds on #963; also the foundation for the coinage port (#638).

Goal

The crash-durable transaction engine runs in the Rust core, so NFT purses and coinage share one implementation on every host.

Scope

• Port the native engine close to 1:1 (hosts/android/feature/transactions, hosts/ios/Packages/DurableTransactions): the durable_tx ledger, completion ladder, recovery pass, submission buckets with backoff, and the per-domain oracle and submission-policy extension points.
• The engine never builds a transaction: it receives a presigned one, or calls back into the domain's submission policy to build or rebuild it.
• Registration joins the caller's database transaction, so domain rows and transaction rows commit together.
• Recovery is driven by the native background schedulers (WorkManager, BGTask) through a runtime-level entry point.

Open questions and risks

• Native recovery reads older blocks through the legacy RPC (chain_getBlockHash, chain_getBlock). The core is built on chainHead_v1, which only serves recent pinned blocks. If the legacy methods are unavailable, recovery after long offline periods needs a redesign.
• Which platform is the reference where lifecycle details differ (recovery start and stop, schedule semantics).

Blocked by #963.