LUD-XX: nodePubkeys for payRequest

[andrerfneves]

Adds an optional nodePubkeys field to the LUD-06 payRequest response, allowing services to advertise the Lightning node public keys that may generate invoices for their LNURL endpoint.

What it does:

• A wallet fetches the LNURL, gets back nodePubkeys alongside the existing fields
• When the wallet receives the invoice from the callback, it extracts the payee node pubkey (from the BOLT11 n field or signature recovery)
• If the invoice's payee doesn't match any advertised pubkey, the wallet shows a non-blocking warning
• If nodePubkeys is absent, behavior is unchanged from LUD-06

Why this matters:
Invoices are increasingly presented through abstractions (embedded browsers, platform relays, aggregated UIs) where the TLS connection is hidden from the user. Wallets also show less invoice detail — often just an amount. This lets the wallet verify invoice provenance at a layer the user can reason about: "this invoice came from one of the nodes this service owns."

Design:

• nodePubkeys is an array of objects (not raw strings) — each entry has a mandatory pubkey field, with room for future fields like alias, since, signature, features defined by later LUDs
• Uses LUD-XX as a placeholder — needs numbering resolution before merge

Dependency: 06 -> XX