feat: 4×64-bit limb representation with Math.multiplyHigh (WIP — breaks build)

Foundation layer for the 4×64-bit limb optimization. This commit rewrites
U256 from IntArray(8) to LongArray(4) and adds platform-specific
Math.multiplyHigh for 64×64→128-bit products:

- JVM: Math.multiplyHigh intrinsic (single IMULH/SMULH instruction)
- Android API 31+: Math.multiplyHigh, fallback to pure Kotlin on older
- Native: pure Kotlin fallback (4 sub-products per multiplyHigh call)

The 4×64 representation reduces inner products from 64 to 16 per field
multiply on JVM, a potential ~1.5-2× speedup on the critical path.

NOTE: This commit intentionally breaks the build — FieldP, ScalarN, Glv,
Point, KeyCodec, Secp256k1, and all tests still expect IntArray(8).
They will be updated in subsequent commits.

https://claude.ai/code/session_01BhU63WUe9AhikZxRdw3Lpg

quartz/src/jvmMain/kotlin/com/vitorpamplona/quartz/utils/secp256k1/MultiplyHigh.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 implementation using Math.multiplyHigh (Java 9+).
+ * The HotSpot JIT compiles this to a single hardware instruction:
+ * IMULH on x86-64, SMULH on ARM64.
+ */
+internal actual fun multiplyHigh(
+    a: Long,
+    b: Long,
+): Long = Math.multiplyHigh(a, b)