lightning: quote the routing fee up front and add a max-send quote endpoint

Users trying to empty their balance over lightning had to guess a maxfee
that covered both an unknown routing fee and the platform fee, and the
0.5% floor we imposed on that guess made an exact "send everything"
impossible. xpay routes with askrene's getroutes, so ask askrene the same
question before we debit and the fee is known exactly.

lib/lnquote.ts mirrors what xpay does in plugins/xpay/xpay.c (v26.06):
a throwaway private layer carrying the invoice's route hints (bolt11) or
blinded paths via xpay's fake destination node (bolt12), then getroutes
over the same layer stack in the same order. The fee is the sum of what
leaves our node minus what the destination receives, rounded up to a
whole sat since that is what xpay's maxfee reserves.

POST /lightning/quote returns the routing fee, the platform fee exactly
as debit() charges it (free-tier credit, ln:nofree rate, none on
sub-accounts) and the total. With `max` it iterates quote / subtract
fees / requote so amount + both fees lands exactly on the balance; an
offer gets a probe invoice then the real one for the solved amount, and
an LNURL caller can pass `ceiling` and a probe invoice the same way.

sendLightning now quotes for every send. With no explicit fee the quote
becomes the maxfee; an explicit fee below the quote is rejected before
any debit instead of failing inside xpay after one; the old 0.5% floor
on explicit fees is gone. If the quote itself fails we fall back to the
heuristic and let xpay try.

finalize rounds the settled fee up to the whole sat that was quoted and
reserved; rounding a 301 msat fee to 0 refunded a sat that the quote had
already accounted for and stranded it in the account.

The prefer-kappa layer is only passed to xpay when it exists on the node;
getroutes rejects unknown layers outright, which broke every regtest send.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

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