Add auto-splice manager background task

Spawns a polling loop from MdkClient::start that watches for
confirmed on-chain funds and automatically splices them into an
existing LSP channel — the symmetric counterpart of the LSP-side
splice-instead-of-open on the inbound path. Default poll is 30s;
tunable via SpliceConfig.

Each tick:
  1. Read list_balances().spendable_onchain_balance_sats. Skip if 0.
  2. Pick the highest-capacity channel where counterparty is the
     configured LSP, is_usable, and has a funding_txo.
  3. Compute fee = ceil(feerate_sat_per_kwu * 400 vB * 4 / 1000),
     using the live ChannelFunding feerate. Skip if balance ≤ fee.
  4. splice_in(amount = balance - fee). Emit SpliceInitiated on Ok;
     log+continue on InsufficientFunds; emit SpliceFailed on other.

The 400 vB constant covers force-close-recovery (N≤3 confirmed
UTXOs) with headroom; the derivation table is in the file's doc
comment. Drop it once we bump ldk-node past 4a2cefc1 and switch to
splice_in_with_all.

In-flight tracking. The is_usable filter is not enough on its own:
once the splice tx confirms and splice_locked propagates, the
channel flips back to usable, but ldk-node's continuously_sync_wallets
only reruns every ~60s. Inside that window list_balances still
reports the pre-splice on-chain value (BDK has not yet observed that
the UTXO was spent), so a tick that lands here will happily re-fire
on the same UTXO. The manager keeps a HashMap<u128, Instant> with a
60s TTL bridging splice completion and the next BDK sync; entries
are evicted at the start of every tick. SkipReason::SpliceAlreadyInFlight
surfaces it in logs.

Idempotency across mdkd restart still relies on BDK locking the
selected UTXOs in the pending splice tx — the in-flight map is in-
memory only.

Author: amackillop