feat(lnurlcash): probe would-be outputs before believing a mutation refusal

The redeem callback is a GET and HTTP stacks retry GETs, so a mint can
execute the first attempt and refuse the repeat as an already-spent input
(LUD-25 says SHOULD replay; real mints refuse). The kit attaches the fresh
output secrets to every refusal - probeMutationOutput tells a landed
mutation from a genuine refusal. The harness wrapper reproduces the retry
against the mock.

src/lnurlcash/ops.testHarness.ts

@@ -69,3 +69,22 @@ export const expectBurned = async (instance: Mint, k1: string): Promise<void> =>
   }
   expect(instance.state.noteState(k1)).toBe('burned')
 }
+
+// a fetch wrapper that answers every /w/cb request with the response to
+// its SECOND, byte-identical send - the HTTP-stack GET retry a real client
+// is subject to: the mint executes the first attempt and (with the mock's
+// default retriedMutation: 'refuse', which is also lnurl-mint's behavior)
+// refuses the repeat as an already-spent input, which is all the caller
+// ever sees
+export const retryingCbFetch = (): typeof fetch => {
+  const impl: typeof fetch = async (input, init) => {
+    const url = typeof input === 'string' ? input : input instanceof URL ? input.href : input.url
+    if (url.includes('/w/cb')) {
+      const first = await fetch(input, init)
+      await first.arrayBuffer()
+      return fetch(input, init)
+    }
+    return fetch(input, init)
+  }
+  return impl
+}

src/lnurlcash/ops/shared.ts

@@ -2,8 +2,9 @@
 // every flow that waits on a payment uses, and the uncertainty type a lost
 // mutation answer surfaces as.
 
-import {fetchInvoiceVerification} from 'lnurlcash-kit'
+import {fetchInvoiceVerification, fetchNoteInfo, withNewK1} from 'lnurlcash-kit'
 import type {LnurlcashOptions, VerifyResult} from 'lnurlcash-kit'
+import {NoteSpentError, NoteUnknownError, noteDeclaredAmount} from 'lnurlcash-kit'
 import type {NewBearer} from '../types'
 
 // a mutation's answer was lost AND the probe could not tell whether it
@@ -36,6 +37,32 @@ export const assertFundOwner = (options: FundOperationOptions): void => {
   options.assertOwner?.()
 }
 
+// what probing a would-be mutation output can tell
+export type OutputProbe = 'live' | 'absent' | 'unknown'
+
+// A refused mutation can still have LANDED: the redeem callback is a GET
+// and HTTP stacks retry GETs, so the service may have executed the first
+// attempt and refused this one as an already-spent input (LUD-25 says a
+// byte-identical retry SHOULD get the original success replayed; real
+// mints refuse instead). The kit attaches the fresh output secrets to
+// every service refusal (newSecretsOf) - so before believing a refusal,
+// probe one would-be output: 'live' proves the mutation landed and the
+// carried secrets are the only money left, 'absent' proves the refusal is
+// genuine, 'unknown' keeps it genuinely ambiguous.
+export const probeMutationOutput = async (
+  noteUrl: string,
+  secret: string,
+  options: LnurlcashOptions = {},
+): Promise<OutputProbe> => {
+  try {
+    await fetchNoteInfo(withNewK1(noteUrl, secret, noteDeclaredAmount(noteUrl) ?? 0), options)
+    return 'live'
+  } catch (err) {
+    if (err instanceof NoteSpentError || err instanceof NoteUnknownError) return 'absent'
+    return 'unknown'
+  }
+}
+
 export type PollOptions = {
   // first delay between checks (doubles each round up to intervalCapMs)
   intervalMs?: number