feat: replace libsodium with pure Kotlin ChaCha20/Poly1305 implementation

Remove LazySodium/JNA/libsodium native dependencies and replace with
pure Kotlin implementations of ChaCha20, HChaCha20, XChaCha20,
Poly1305 MAC, and XChaCha20-Poly1305 AEAD. This eliminates platform-
specific native binaries while maintaining full NIP-44 compatibility.

- Add ChaCha20Core with RFC 8439 block function, IETF stream XOR,
  HChaCha20, and XChaCha20
- Add Poly1305 MAC (RFC 8439 §2.5)
- Add XChaCha20Poly1305 AEAD encrypt/decrypt
- Convert LibSodiumInstance from expect/actual to commonMain object
- Delete platform-specific LibSodiumInstance actuals (android/jvm/ios)
- Remove iOS native interop (cinterop, linker opts, .a libraries)
- Remove lazysodium-java, lazysodium-android, JNA from build deps
- Clean up ProGuard rules (remove JNA/LazySodium keep rules)
- Add RFC 8439 + libsodium test vectors for ChaCha20 and XChaCha20
- Update benchmark to use simplified ChaCha20 API

https://claude.ai/code/session_01YHBwqnb3Z7Q7PX31tJ2G3i

benchmark/src/androidTest/java/com/vitorpamplona/quartz/benchmark/ChaCha20Benchmark.kt

@@ -52,36 +52,16 @@ class ChaCha20Benchmark {
     }
 
     @Test
-    fun encryptLibSodium() {
+    fun encrypt() {
         benchmarkRule.measureRepeated {
-            chaCha.encryptLibSodium(padded, messageKeys.chachaNonce, messageKeys.chachaKey)
+            chaCha.encrypt(padded, messageKeys.chachaNonce, messageKeys.chachaKey)
         }
     }
 
-    /*
-        Removed in the conversion to KMP
     @Test
-    fun encryptNative() {
+    fun decrypt() {
         benchmarkRule.measureRepeated {
-            chaCha.encryptNative(padded, messageKeys.chachaNonce, messageKeys.chachaKey)
+            chaCha.decrypt(padded, messageKeys.chachaNonce, messageKeys.chachaKey)
         }
     }
-     */
-
-    @Test
-    fun decryptLibSodium() {
-        benchmarkRule.measureRepeated {
-            chaCha.decryptLibSodium(padded, messageKeys.chachaNonce, messageKeys.chachaKey)
-        }
-    }
-
-    /*
-    Removed in the conversion to KMP
-    @Test
-    fun decryptNative() {
-        benchmarkRule.measureRepeated {
-            chaCha.decryptNative(padded, messageKeys.chachaNonce, messageKeys.chachaKey)
-        }
-    }
-     */
 }