Summary

This PR implements the core Paykit integration module for Bitkit Android:

• PaykitManager: Singleton managing PaykitClient lifecycle and executor registration. Maps LDK Network to Paykit network configs (MAINNET/TESTNET/REGTEST/SIGNET). Uses Mutex for thread-safe coroutine initialization.

• BitkitBitcoinExecutor: Implements BitcoinExecutorFFI, bridging LightningRepo.sendOnChain() to Paykit's onchain payment interface. Uses runBlocking(Dispatchers.IO) with withTimeout for coroutine-to-sync FFI bridging.

• BitkitLightningExecutor: Implements LightningExecutorFFI, bridging LightningRepo.payInvoice() to Paykit. Includes payment completion polling via getPayments(), SHA-256 preimage verification, and hex encoding utilities.

• PaykitIntegrationHelper: Convenience methods for setup and payment execution, with both suspend and callback-based async APIs.

Key Features

1. Network Configuration Mapping: Automatically maps Bitkit's Env.network to Paykit's network enums
2. Coroutine-to-Sync Bridging: Uses runBlocking with Dispatchers.IO and timeouts for sync FFI calls
3. Payment Completion Polling: Polls LightningRepo.getPayments() to retrieve payment status and preimage
4. Thread Safety: Uses Kotlin Mutex for thread-safe initialization
5. Dagger/Hilt Compatible: PaykitManager annotated with @Singleton and @Inject

Prerequisites

PaykitMobile UniFFI bindings must be generated and linked before these components are fully functional. TODOs in the code mark where binding code should be uncommented.

Test plan

• Generate PaykitMobile bindings with cargo build --features mobile
• Add jniLibs to Gradle configuration
• Uncomment FFI binding code in PaykitManager and executors
• Test PaykitIntegrationHelper.setup(lightningRepo) during app startup
• Verify Lightning payments return preimage proof
• Verify onchain payments return txid