refactor(quartz): migrate in-tree C secp256k1 to libschnorr256k1-kmp

The custom C secp256k1 implementation under quartz/src/main/c/ was never
wired into Gradle (no externalNativeBuild block) and only existed for the
3-way benchmark + cross-validation test against ACINQ on JVM/Android.
The C library has been split out to vitorpamplona/libschnorr256k1{,-kmp},
so the in-repo copy is dead weight.

This change replaces it with the published Maven Central artifact
`com.vitorpamplona:schnorr256k1-kmp:1.0.0`, scoped to test/benchmark
configurations only — production crypto continues to use ACINQ
secp256k1-kmp on JVM/Android and the pure-Kotlin Secp256k1 on native.

The Android AAR ships libschnorr256k1_jni.so for arm64-v8a and x86_64,
so the Android benchmark now exercises the C row automatically (no more
manual build_android.sh). On JVM the .so still has to be installed by the
developer; the cross-validation test and triple-benchmark gracefully skip
the C row when System.loadLibrary fails, matching prior behavior.

Verified on JVM: all 13 secp256k1 jvmTest classes pass (188 tests), and
ACINQ vs pure-Kotlin benchmark numbers match the documented baseline
within sandbox noise (verifySchnorr ~15k ops/s, signSchnorr cached
~33k ops/s, pubkeyCreate+Compress ~36k ops/s).

Removes ~5,100 LOC: quartz/src/main/c/ (4,500), Secp256k1InstanceC.*
expect/actual shim (560), Secp256k1C JNI declaration object.

https://claude.ai/code/session_01KnvpK2amcVZKfFiZJvHjVe

quartz/src/androidMain/kotlin/com/vitorpamplona/quartz/utils/Secp256k1InstanceC.android.kt

@@ -1,187 +0,0 @@
-/*
- * 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
-
-/**
- * Android actual: delegates to Secp256k1C JNI binding class.
- * The native library is packaged as libsecp256k1_amethyst_jni.so
- * in the APK's jniLibs (built via CMake in quartz/src/main/c/).
- */
-object Secp256k1C {
-    private var loaded = false
-
-    fun ensureLoaded() {
-        if (!loaded) {
-            System.loadLibrary("secp256k1_amethyst_jni")
-            nativeInit()
-            loaded = true
-        }
-    }
-
-    @JvmStatic external fun nativeInit()
-
-    @JvmStatic external fun nativePubkeyCreate(seckey: ByteArray): ByteArray?
-
-    @JvmStatic external fun nativePubkeyCompress(pubkey: ByteArray): ByteArray?
-
-    @JvmStatic external fun nativeSecKeyVerify(seckey: ByteArray): Boolean
-
-    @JvmStatic external fun nativeSchnorrSign(
-        msg: ByteArray,
-        seckey: ByteArray,
-        auxrand: ByteArray?,
-    ): ByteArray?
-
-    @JvmStatic external fun nativeSchnorrSignXOnly(
-        msg: ByteArray,
-        seckey: ByteArray,
-        xonlyPub: ByteArray,
-        auxrand: ByteArray?,
-    ): ByteArray?
-
-    @JvmStatic external fun nativeSchnorrVerify(
-        sig: ByteArray,
-        msg: ByteArray,
-        pub: ByteArray,
-    ): Boolean
-
-    @JvmStatic external fun nativeSchnorrVerifyFast(
-        sig: ByteArray,
-        msg: ByteArray,
-        pub: ByteArray,
-    ): Boolean
-
-    @JvmStatic external fun nativeSchnorrVerifyBatch(
-        pub: ByteArray,
-        sigs: Array<ByteArray>,
-        msgs: Array<ByteArray>,
-    ): Boolean
-
-    @JvmStatic external fun nativePrivKeyTweakAdd(
-        seckey: ByteArray,
-        tweak: ByteArray,
-    ): ByteArray?
-
-    @JvmStatic external fun nativePubKeyTweakMul(
-        pubkey: ByteArray,
-        tweak: ByteArray,
-    ): ByteArray?
-
-    @JvmStatic external fun nativeEcdhXOnly(
-        xonlyPub: ByteArray,
-        scalar: ByteArray,
-    ): ByteArray?
-
-    @JvmStatic external fun nativeSha256(data: ByteArray): ByteArray?
-}
-
-actual object Secp256k1InstanceC {
-    actual fun init() = Secp256k1C.ensureLoaded()
-
-    actual fun compressedPubKeyFor(privKey: ByteArray): ByteArray {
-        Secp256k1C.ensureLoaded()
-        val pub65 = Secp256k1C.nativePubkeyCreate(privKey) ?: error("Invalid private key")
-        return Secp256k1C.nativePubkeyCompress(pub65) ?: error("Compression failed")
-    }
-
-    actual fun isPrivateKeyValid(il: ByteArray): Boolean {
-        Secp256k1C.ensureLoaded()
-        return Secp256k1C.nativeSecKeyVerify(il)
-    }
-
-    actual fun signSchnorr(
-        data: ByteArray,
-        privKey: ByteArray,
-        nonce: ByteArray?,
-    ): ByteArray {
-        Secp256k1C.ensureLoaded()
-        return Secp256k1C.nativeSchnorrSign(data, privKey, nonce) ?: error("Sign failed")
-    }
-
-    actual fun signSchnorrWithXOnlyPubKey(
-        data: ByteArray,
-        privKey: ByteArray,
-        xOnlyPubKey: ByteArray,
-        nonce: ByteArray?,
-    ): ByteArray {
-        Secp256k1C.ensureLoaded()
-        return Secp256k1C.nativeSchnorrSignXOnly(data, privKey, xOnlyPubKey, nonce) ?: error("Sign failed")
-    }
-
-    actual fun verifySchnorr(
-        signature: ByteArray,
-        hash: ByteArray,
-        pubKey: ByteArray,
-    ): Boolean {
-        Secp256k1C.ensureLoaded()
-        return Secp256k1C.nativeSchnorrVerify(signature, hash, pubKey)
-    }
-
-    actual fun verifySchnorrFast(
-        signature: ByteArray,
-        hash: ByteArray,
-        pubKey: ByteArray,
-    ): Boolean {
-        Secp256k1C.ensureLoaded()
-        return Secp256k1C.nativeSchnorrVerifyFast(signature, hash, pubKey)
-    }
-
-    actual fun verifySchnorrBatch(
-        pubKey: ByteArray,
-        signatures: List<ByteArray>,
-        messages: List<ByteArray>,
-    ): Boolean {
-        Secp256k1C.ensureLoaded()
-        return Secp256k1C.nativeSchnorrVerifyBatch(
-            pubKey,
-            signatures.toTypedArray(),
-            messages.toTypedArray(),
-        )
-    }
-
-    actual fun privateKeyAdd(
-        first: ByteArray,
-        second: ByteArray,
-    ): ByteArray {
-        Secp256k1C.ensureLoaded()
-        return Secp256k1C.nativePrivKeyTweakAdd(first, second) ?: error("Tweak add failed")
-    }
-
-    actual fun pubKeyTweakMulCompact(
-        pubKey: ByteArray,
-        privateKey: ByteArray,
-    ): ByteArray {
-        Secp256k1C.ensureLoaded()
-        val compressedPub = ByteArray(33)
-        compressedPub[0] = 0x02
-        pubKey.copyInto(compressedPub, 1, 0, 32)
-        val result = Secp256k1C.nativePubKeyTweakMul(compressedPub, privateKey) ?: error("Tweak mul failed")
-        return result.copyOfRange(1, 33)
-    }
-
-    actual fun ecdhXOnly(
-        xOnlyPub: ByteArray,
-        scalar: ByteArray,
-    ): ByteArray {
-        Secp256k1C.ensureLoaded()
-        return Secp256k1C.nativeEcdhXOnly(xOnlyPub, scalar) ?: error("ECDH failed")
-    }
-}