Add swap-in wallet and splice-in via Electrum backend

[matthiasdebernardini]

Summary

• Switches the blockchain backend from MempoolSpaceClient to ElectrumClient/ElectrumWatcher, enabling peer.swapInWallet and peer.finalWallet (which require IElectrumClient)
• Adds three new API endpoints: GET /getswapinaddress, GET /swapinwalletbalance, POST /splicein
• Adds corresponding CLI commands: phoenix-cli getswapinaddress, swapinwalletbalance, splicein
• Calls peer.startWatchSwapInWallet() after both Electrum and peer connections are established, enabling automatic swap-in to channel
• Adds --electrum-server CLI option with per-chain defaults, deprecates --mempool-space-url and --mempool-space-polling-interval-minutes

This is based on the approach from #69 but ported to the current codebase and API surface.

Motivation

Phoenixd currently uses MempoolSpaceClient which only implements IClient, not IElectrumClient. This means peer.swapInWallet and peer.finalWallet are always null — users cannot receive on-chain funds and splice them into channels. The Phoenix mobile app uses ElectrumClient and has full swap-in support; this PR brings phoenixd to parity.

Changes

File	Change
Phoenixd.kt	Replace MempoolSpace with ElectrumClient/Watcher, add --electrum-server option, Electrum connection loop, startWatchSwapInWallet() call, deprecate mempool options
Api.kt	Add getswapinaddress, swapinwalletbalance (read-only), splicein (full-access) endpoints
JsonSerializers.kt	Add SwapInAddress and SwapInWalletBalance response types
PhoenixCli.kt	Add GetSwapInAddress, GetSwapInWalletBalance, SpliceIn CLI commands

Mainnet E2E Test Results

Tested on mainnet with real sats (22,333 sat across 3 deposits):

• ✅ Electrum connection to electrum.acinq.co:50002
• ✅ Swap-in address generation with auto-rotation (taproot addresses, derivation indices)
• ✅ Near-instant UTXO detection via Electrum push notifications
• ✅ Confirmation tracking (unconfirmed → weaklyConfirmed → deeplyConfirmed)
• ✅ Automatic channel request triggered at deeplyConfirmed threshold
• ✅ sendtoaddress splice-out worked (19,000 sat sent on-chain, tx 15539c9a61114c3bb2940dce83badaa58c540ba8064f553cf416ee1761c41969)
• ✅ All existing endpoints unaffected

Known Issue: Feerate Estimation

The Electrum server's blockchain.estimatefee returned very low values (1 sat/vB), causing fundingFeerate=253 sat/kw. The ACINQ LSP aborted channel creation during interactive-tx with channel funding error. The channel was ultimately opened via Phoenix mobile (same seed), after which phoenixd synced the state and successfully performed splice-out.

Possible fixes:

• Add a minimum feerate floor
• Use a hybrid approach (Electrum for address monitoring, mempool.space for fee estimation)
• Investigate why electrum.acinq.co returns low fee estimates

Other Notes

• Default testnet Electrum server testnet.acinq.co:51002 doesn't resolve — needs a working default
• Channel opening fees are ~21k sat, so minimum recommended deposit is ~25k sat
• No build.gradle.kts changes needed — ElectrumClient/ElectrumWatcher are part of lightning-kmp-core (already a dependency)

Test plan

• Build compiles cleanly with zero warnings
• GET /getswapinaddress returns valid taproot address with derivation index
• GET /swapinwalletbalance correctly reports total and unconfirmed balances
• Address auto-rotates when previous address receives funds
• UTXO confirmation tracking works (unconfirmed → weakly → deeply)
• Auto channel creation triggers at deeplyConfirmed
• POST /sendtoaddress splice-out works from an open channel
• Channel creation succeeds autonomously (blocked by feerate issue)
• POST /splicein manual splice-in (not tested — auto flow triggered first)
• Testnet with working Electrum server

🤖 Generated with Claude Code