lnurlp: bind direct receives to the metadata description hash (LUD-18 payerdata)

[singap2002-code]

Description

Fixes #3220.

Since ca3b05e, LN-address payments to users with proxyReceive disabled are received directly through their attached wallet via createExternalReceiveInvoice(). When the payer attaches LUD-18 payerdata, pay.js computes descriptionHash = sha256(metadata + payerdata) and passes it down the direct path with requireDescriptionHash: !!nostr — i.e. unenforced for LUD-18 requests.

Consequences:

1. createBolt11FromWalletProtocols() only filters protocolSupportsDescriptionHash when requireDescriptionHash is true, so hash-incapable protocols (LN_ADDR, LNBITS, PHOENIXD, BLINK, CLINK) mint an invoice with a plain description memo;
2. the post-mint hash verification is skipped as well.

The payer therefore receives an invoice whose description does not hash to what the service recorded, while LUD-06 wallets bind the invoice to the fetched metadata — a mismatch that spec-conforming wallets can reject. The proxy path doesn't have this problem because the wrapped payer-facing invoice is actually minted with the descriptionHash tag.

Fix

Bind direct receives to the plain-metadata description hash (LUD-18 payer identity is persisted separately via lud18Data), and keep the zap-request hash for NIP-57, which is already enforced via requireDescriptionHash: !!nostr:

descriptionHash: nostr ? descriptionHash : metadataDescriptionHash,
descriptionHashPreimage: nostr ? descriptionHashPreimage : metadata,

This keeps direct receives working for every protocol (no new hard requirement for hash-capable wallets) and makes the payer-facing invoice consistent with what wallets verify.

Additional Context

• Companion to NIP-57 receipt worker crashes (receipt silently lost) when the zap request has no relays tag #3218 / draft worker: publish NIP-57 receipts even when zap request lacks relays tag #3219 (same commit area, NIP-57 path).
• Marked as draft per CONTRIBUTING until a maintainer confirms the intended semantics for LUD-18 on direct receives (alternative: enforce requireDescriptionHash for payerdata too — but that would break LUD-18 payments for users whose only protocols lack description-hash support).

Checklist

• I have read the contributing guidelines
• I have performed a self-review of my code
• My changes generate no new warnings/errors (syntax verified; change is confined to one branch argument in pay.js)
• I confirm this contribution was prepared with AI assistance; all code references were verified against master (d4aaf0a) and the specs (LUD-06, LUD-18)