perf: make uLt() platform-specific to avoid JVM regression

The inline uLt() using XOR trick in commonMain regressed JVM by ~30%
because HotSpot's Long.compareUnsigned is a JIT intrinsic (single
unsigned CMP + SETB), while the XOR trick generates 2 extra XOR insns
that HotSpot doesn't optimize away.

Convert uLt to expect/actual:
- JVM: Long.compareUnsigned (HotSpot intrinsic)
- Android: XOR trick (avoids ULong.constructor-impl NOOP calls)
- Native: XOR trick (no JVM intrinsics available)

JVM verify: 2.1x → 1.5x (restored, slightly better than 1.6x baseline)
Android: unchanged (still uses XOR trick)

https://claude.ai/code/session_01EMY5RnXb9rnsyU2KbXrSaY

quartz/src/androidMain/kotlin/com/vitorpamplona/quartz/utils/secp256k1/UnsignedCompare.android.kt

@@ -0,0 +1,32 @@
+/*
+ * Copyright (c) 2025 Vitor Pamplona
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to use,
+ * copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the
+ * Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
+ * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+package com.vitorpamplona.quartz.utils.secp256k1
+
+/**
+ * Android: XOR-with-MIN_VALUE trick avoids the ULong.constructor-impl
+ * NOOP invokestatic calls that Kotlin's toULong() generates. Produces
+ * pure arithmetic bytecode (lxor, lcmp) with zero method calls.
+ * ART's JIT compiles this to EOR + CMP + CSET on ARM64.
+ */
+internal actual fun uLt(
+    a: Long,
+    b: Long,
+): Boolean = (a xor Long.MIN_VALUE) < (b xor Long.MIN_VALUE)