fix: replace ThreadLocal with KMP-compatible ScratchLocal expect/actual

ThreadLocal.withInitial is a JVM-only API that doesn't compile on
Kotlin/Native or iOS targets. Replace all usages in commonMain
(FieldP.kt, Point.kt) with a new ScratchLocal expect/actual:

- jvmMain/androidMain: delegates to java.lang.ThreadLocal (same behavior)
- nativeMain: holds value directly (Kotlin/Native coroutines are
  cooperative, scratch buffers don't need thread isolation)

https://claude.ai/code/session_01BhU63WUe9AhikZxRdw3Lpg

quartz/src/androidMain/kotlin/com/vitorpamplona/quartz/utils/secp256k1/ScratchLocal.android.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
+
+/** Android: delegates to java.lang.ThreadLocal for per-thread scratch buffers. */
+internal actual class ScratchLocal<T> actual constructor(
+    initializer: () -> T,
+) {
+    private val tl = ThreadLocal.withInitial(initializer)
+
+    actual fun get(): T = tl.get()
+}