feat: add batch verify to Android benchmark + expose in Secp256k1InstanceOurs

Add verifySchnorrBatch to Secp256k1InstanceOurs wrapper and add
Android benchmark tests for batch(8) and batch(16).

To get per-event cost: ns/op ÷ batchSize.
JVM benchmark showed 4-7× speedup over individual verify.
Android results TBD.

https://claude.ai/code/session_01EMY5RnXb9rnsyU2KbXrSaY

quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/utils/Secp256k1InstanceOurs.kt

@@ -72,6 +72,16 @@ object Secp256k1InstanceOurs {
         pubKey: ByteArray,
     ): Boolean = Secp256k1.verifySchnorrFast(signature, hash, pubKey)
 
+    /**
+     * Batch-verify multiple signatures from the same pubkey.
+     * Uses scalar+point summation — one mulDoubleG for the whole batch.
+     */
+    fun verifySchnorrBatch(
+        pubKey: ByteArray,
+        signatures: List<ByteArray>,
+        messages: List<ByteArray>,
+    ): Boolean = Secp256k1.verifySchnorrBatch(pubKey, signatures, messages)
+
     fun privateKeyAdd(
         first: ByteArray,
         second: ByteArray,