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. is_usable alone is not a sound signal: a channel
mid-splice can still report is_usable=true after we've sent our 0-conf
splice_locked but before the LSP returns theirs (gated on
inbound_splice_minimum_depth confirmations). The manager keeps a
HashMap<UCID, InFlight { initial_funding_txo, promoted_at }> driven
by ChannelDetails::funding_txo. While the channel's active funding
still equals the outpoint captured at splice_in time, the splice is
in flight and the channel is excluded from the candidate pool
(rust-lightning only swaps self.funding once both sides have
exchanged splice_locked, in maybe_promote_splice_funding). Once
funding_txo flips, the entry is held for BDK_RESYNC_GRACE (60s) so
the next tick doesn't race continuously_sync_wallets and re-fire on
a UTXO BDK hasn't yet observed as spent. 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