fix(cashu): drop the JVM-only HashMap.putIfAbsent from commonMain

CashuMintOperations.checkProofStates used java.util.HashMap.putIfAbsent,
which the commonMain metadata compile rejects. The map is local and
single-threaded, so an explicit contains-then-set keeps the same
first-secret-wins behaviour.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S7FuNBSKiyVecARSoE4B9P

quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip60Cashu/mintApi/CashuMintOperations.kt

@@ -687,10 +687,13 @@ class CashuMintOperations(
         val ys = ArrayList<String>(proofs.size)
         proofs.forEach { proof ->
             val y = Bdhke.hashToCurveCompressed(proof.secret.encodeToByteArray()).toHexKey()
-            // putIfAbsent: two proofs can legitimately carry the same secret
+            // First one wins: two proofs can legitimately carry the same secret
             // (a duplicated kind:7375 the dedup pass hasn't retired yet), and
             // they map to the same state anyway.
-            if (secretByY.putIfAbsent(y, proof.secret) == null) ys.add(y)
+            if (y !in secretByY) {
+                secretByY[y] = proof.secret
+                ys.add(y)
+            }
         }
 
         val out = HashMap<String, ProofState>(secretByY.size)