feat: expose verifySchnorrFast in Secp256k1InstanceOurs + Android benchmark

Wire verifySchnorrFast (x-check only, no y-parity inversion) into:
- Secp256k1InstanceOurs wrapper for app-level usage
- Android benchmark as verifySchnorrFastOurs for Pixel 8 measurement

Expected: ~15% faster than verifySchnorrOurs (~100μs vs ~120μs on Pixel 8)

https://claude.ai/code/session_01EMY5RnXb9rnsyU2KbXrSaY

benchmark/src/androidTest/java/com/vitorpamplona/quartz/benchmark/Secp256k1Benchmark.kt

@@ -123,6 +123,13 @@ class Secp256k1Benchmark {
         }
     }
 
+    @Test
+    fun verifySchnorrFastOurs() {
+        benchmarkRule.measureRepeated {
+            assertTrue(Secp256k1InstanceOurs.verifySchnorrFast(signature, msg32, xOnlyPub))
+        }
+    }
+
     @Test
     fun signSchnorrOurs() {
         benchmarkRule.measureRepeated {