Add initial destination-aware max_sendable

[amackillop]

Bump ldk-node to the rev that lets find_route take a fully-built
RouteParameters and use it from the Some(_) branch of
MdkClient::max_sendable. The reported fee budget is built from
route.get_total_fees(), which is what LDK itself would charge for
a send of that size. Slightly conservative: total_fees is computed
from the full balance so with the sent amount they will be less.
This provides a buffer and could be tweaked by iterating the estimate.

On top of total_fees the budget applies a multiplier. The chosen route
can fail at send time and LDK retries along a more expensive path. A
multiplier scales with the original route's cost so retries can
actually reach those alternatives. Exposed as
route_retry_fee_multiplier_bps so operators can trade reported max
sendable for payment success rate. Starting low to minimise headroom
left on the table; bump up if production retries surface
"insufficient fee budget" failures too often.

FixedAmount returns Err(FixedAmount { amount_msat }) because there's
nothing to estimate when the payee dictates the amount; the variant
carries the amount so callers don't re-extract it.

BOLT12 offers, LNURL-pay, and HRN destinations fall back to the
simple buffer for now. BOLT12 needs the invoice (not the offer) for
from_bolt12_invoice, which depends on the invoice fetch deferred
upstream. LNURL-pay and HRN need to resolve into a concrete invoice
first; that resolution will move into this module shortly.

BOLT11 round-trips through its bech32 string: bitcoin-payment-
instructions pins upstream rust-lightning's Bolt11Invoice while
ldk-node pulls the moneydevkit fork. Wire-compatible, distinct types.
A re-parse failure falls back to Buffer rather than panicking.