Add Node::get_max_splice_in_amount accessor

[amackillop]

Computes the wallet's maximum splice-in contribution for a channel via a dry-run drain transaction at the live ChannelFunding fee rate, preserving the anchor-channels reserve as a wallet change output. Plugs directly into splice_in as splice_amount_sats, so callers can splice everything without computing fees themselves.

Backports upstream's Node::splice_in_with_all (4a2cefc) without the splice API rework (d9336f2) that sits between it and our pin. A pin bump is painful because of LSPS4 fork rebases on rust-lightning, so we replicate just the amount computation here and leave splice_in unchanged. Once we do bump past d9336f2, this accessor is the only thing that gets replaced with splice_in_with_all, and mdkd's caller follows.

The drain pattern mirrors Wallet::send_to_address's AllRetainingReserve branch: drain_wallet plus drain_to(funding script), with an add_recipient for cur_anchor_reserve_sats when non-dust so the reserve survives as wallet change. Wallet net contribution = drain output value minus foreign input total, which is what splice_in adds to the channel.

The motivating caller is mdkd's auto-splice manager. It previously computed a static 400 vB upper-bound fee, capped at N=3 client UTXOs, so consolidation flows had to close channels one or two at a time. With this accessor mdkd uses BDK's exact coin selection and that caveat goes away.

A pure-mdkd alternative was considered: binary-search splice_in calls until one succeeds. We didn't take it because Error::ChannelSplicingFailed collapses BDK selection failure and genuine peer rejection into one variant, so the retry loop can't tell "shrink the amount" from "give up".