fix(quartz): KMP-safe @Volatile import in Cashu warmup flags

`@Volatile` in commonMain resolves to `kotlin.jvm.Volatile` by default,
which doesn't exist on iOS targets. Two new NIP-60 Cashu files use
`@Volatile` without an explicit import, breaking
`:quartz:compileKotlinIosSimulatorArm64`:

  Bdhke.kt:577:6 Unresolved reference 'Volatile'.
  MintApiSerializerWarmup.kt:74:6 Unresolved reference 'Volatile'.

Add `import kotlin.concurrent.Volatile` to both files. Same pattern as
6f1292bfc (Note.kt) — semantics unchanged on JVM/Android, now also
resolves on iOS.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip60Cashu/bdhke/Bdhke.kt

@@ -31,6 +31,7 @@ import com.vitorpamplona.quartz.utils.secp256k1.ScalarN
 import com.vitorpamplona.quartz.utils.secp256k1.Secp256k1
 import com.vitorpamplona.quartz.utils.secp256k1.U256
 import com.vitorpamplona.quartz.utils.sha256.sha256
+import kotlin.concurrent.Volatile
 
 /**
  * Blind Diffie-Hellman Key Exchange (BDHKE) — the cryptographic primitive

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

@@ -21,6 +21,7 @@
 package com.vitorpamplona.quartz.nip60Cashu.mintApi
 
 import kotlinx.serialization.json.Json
+import kotlin.concurrent.Volatile
 
 /**
  * Pre-warms kotlinx.serialization for the hot mint-API DTOs.