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)

quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/utils/secp256k1/U256.kt

@@ -46,26 +46,22 @@ package com.vitorpamplona.quartz.utils.secp256k1
 // =====================================================================================
 
 /**
- * Unsigned less-than comparison without ULong inline class overhead.
+ * Unsigned less-than comparison, platform-optimized.
  *
- * Kotlin's `uLt(a, b)` generates 2 invokestatic calls to
- * ULong.constructor-impl (NOOPs that return the input unchanged) plus
- * Long.compareUnsigned per comparison. On ART, these extra invokestatic
- * calls add ~2-3ns each × ~18,000 comparisons per verify = ~36-54μs.
+ * On JVM (HotSpot): uses Long.compareUnsigned which is a JIT intrinsic,
+ * compiling to a single unsigned CMP + SETB instruction.
  *
- * This inline function uses the XOR-with-MIN_VALUE trick directly,
- * producing pure arithmetic bytecode with ZERO method calls:
- *   lload a, ldc MIN_VALUE, lxor, lload b, ldc MIN_VALUE, lxor, lcmp, ifge
+ * On Android (ART): uses XOR-with-MIN_VALUE trick to avoid the
+ * ULong.constructor-impl NOOP invokestatic calls that Kotlin's toULong()
+ * generates (~17,800 per verify). Produces pure arithmetic bytecode with
+ * zero method calls.
  *
- * vs the toULong() path:
- *   lload a, invokestatic ULong.constructor-impl, lload b,
- *   invokestatic ULong.constructor-impl, invokestatic Long.compareUnsigned, ifge
+ * On Native: uses XOR-with-MIN_VALUE (no JVM intrinsics available).
  */
-@Suppress("NOTHING_TO_INLINE")
-internal inline fun uLt(
+internal expect fun uLt(
     a: Long,
     b: Long,
-): Boolean = (a xor Long.MIN_VALUE) < (b xor Long.MIN_VALUE)
+): Boolean
 
 /**
  * Raw 256-bit unsigned integer arithmetic using 4×64-bit limbs.

quartz/src/jvmMain/kotlin/com/vitorpamplona/quartz/utils/secp256k1/UnsignedCompare.jvm.kt

@@ -0,0 +1,31 @@
+/*
+ * 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
+
+/**
+ * JVM: Long.compareUnsigned is a HotSpot JIT intrinsic that compiles
+ * to a single unsigned CMP + SETB instruction. Much faster than the
+ * XOR trick on HotSpot.
+ */
+internal actual fun uLt(
+    a: Long,
+    b: Long,
+): Boolean = java.lang.Long.compareUnsigned(a, b) < 0

quartz/src/nativeMain/kotlin/com/vitorpamplona/quartz/utils/secp256k1/UnsignedCompare.native.kt

@@ -0,0 +1,30 @@
+/*
+ * 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
+
+/**
+ * Native: XOR-with-MIN_VALUE trick (no JVM intrinsics available).
+ * Kotlin/Native AOT compiles this to efficient unsigned compare.
+ */
+internal actual fun uLt(
+    a: Long,
+    b: Long,
+): Boolean = (a xor Long.MIN_VALUE) < (b xor Long.MIN_VALUE)