fix(quartz): hash empty input on Apple without crashing

sha256() and sha256Into() in Sha256.apple.kt pinned the input and took
addressOf(0), which throws ArrayIndexOutOfBoundsException on Kotlin/Native
when the array is empty. So sha256(ByteArray(0)) crashed on iOS/macOS, and
test-quartz-ios failed 7 OversizedTransferTest cases that hash "" (red on main
too). CommonCrypto's CC_SHA256 accepts a null data pointer for a zero-length
input, so pass null when the array is empty.

Adds Sha256Test with FIPS 180-2 vectors (empty, "abc", prefix hashing through
sha256Into) so every platform's actual is checked, not only through CEP-22.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015VvJ8XpeYqBe8bYT3bJ7UD

quartz/src/appleMain/kotlin/com/vitorpamplona/quartz/utils/sha256/Sha256.apple.kt

@@ -34,7 +34,8 @@ actual fun sha256(data: ByteArray): ByteArray {
     data.usePinned { inputPinned ->
         digest.usePinned { digestPinned ->
             CC_SHA256(
-                inputPinned.addressOf(0),
+                // addressOf(0) throws on an empty array; CommonCrypto accepts null for a 0-byte input.
+                if (data.isEmpty()) null else inputPinned.addressOf(0),
                 data.size.convert(),
                 digestPinned.addressOf(0),
             )
@@ -53,7 +54,8 @@ actual fun sha256Into(
     data.usePinned { inputPinned ->
         out.asUByteArray().usePinned { digestPinned ->
             CC_SHA256(
-                inputPinned.addressOf(0),
+                // addressOf(0) throws on an empty array; CommonCrypto accepts null for a 0-byte input.
+                if (data.isEmpty()) null else inputPinned.addressOf(0),
                 len.convert(),
                 digestPinned.addressOf(0),
             )

quartz/src/commonTest/kotlin/com/vitorpamplona/quartz/utils/sha256/Sha256Test.kt

@@ -0,0 +1,58 @@
+/*
+ * 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.sha256
+
+import kotlin.test.Test
+import kotlin.test.assertEquals
+
+/**
+ * Known-answer vectors (FIPS 180-2) for every platform's sha256 actual. The empty
+ * input case guards the Apple actual, which used to pin `addressOf(0)` on a
+ * zero-length array and throw ArrayIndexOutOfBoundsException.
+ */
+class Sha256Test {
+    private val emptyDigest = "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855"
+    private val abcDigest = "ba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad"
+
+    private fun ByteArray.hex() = joinToString("") { (it.toInt() and 0xFF).toString(16).padStart(2, '0') }
+
+    @Test
+    fun hashesEmptyInput() {
+        assertEquals(emptyDigest, sha256(ByteArray(0)).hex())
+    }
+
+    @Test
+    fun hashesAbc() {
+        assertEquals(abcDigest, sha256("abc".encodeToByteArray()).hex())
+    }
+
+    @Test
+    fun sha256IntoHashesEmptyInput() {
+        assertEquals(emptyDigest, sha256Into(ByteArray(32), ByteArray(0)).hex())
+    }
+
+    @Test
+    fun sha256IntoHashesOnlyThePrefix() {
+        val out = ByteArray(32)
+        assertEquals(abcDigest, sha256Into(out, "abcdef".encodeToByteArray(), 3).hex())
+        assertEquals(emptyDigest, sha256Into(out, "abcdef".encodeToByteArray(), 0).hex())
+    }
+}