nwc: conform pay/receive to the BIP-321 methods spec (nwc#2)

Tightens our implementation of the BIP-321-based NWC methods proposed
in https://github.com/nostr-wallet-connect/nwc/pull/2 — the successor
to pay_invoice/make_invoice that clients are asked to transition to
for bolt12 zaps in https://github.com/nostr-protocol/nips/pull/2421:

- reject cross-network payment instructions before paying
  (UNSUPPORTED_NETWORK) instead of failing after the debit
- reject unknown req- BIP-321 params, including req-pop since we
  can't open proof-of-payment callbacks
- reject a request amount that conflicts with the instruction amount
- payer_note is delivered or rejected, never dropped: bolt11+note is
  rejected, internal settlements hand the note over as the payment memo
- receive mints a per-request bolt12 offer carrying the requested
  description/amount instead of reusing the standing amountless offer
- get_info reports the node's actual network instead of "mainnet"

Co-Authored-By: Claude Fable 5 <[email protected]>

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