fix(lnurlcash): assert named claims land at the wallet's own secret

Review feedback: bind the claim answer to the secret that was polled
before building a note from it. The kit already guarantees this -
claimMintedNote returns the queried secret by construction and
fetchNoteInfo rejects a mismatched k1 echo - so the assert cannot trip
through the kit (and is therefore not separately testable); it exists so
a future kit regression fails loudly instead of tracking a wrong secret.

src/lnurlcash/ops/mint.ts

@@ -245,6 +245,13 @@ export const claimFromSecret = async (
     // is lying about the note it just credited
     throw new Error('The mint reports the freshly minted note as already spent.')
   }
+  if (claim.k1 !== target.noteSecret) {
+    // belt and suspenders: the kit's claimMintedNote returns the queried
+    // secret by construction and fetchNoteInfo rejects a mismatched echo,
+    // so this cannot trip through the kit - if it ever does, building a
+    // note from the answer would track the wrong secret
+    throw new Error('The mint answered the claim for a different note secret.')
+  }
   if (claim.amountMsat === null || !claim.callback) {
     throw new Error('The mint did not return the minted note details.')
   }