feat(sdk): add Swift and Kotlin mobile client foundations with push token proto

Swift SDK: Swift Package wrapping libquicproquo_ffi with QpqClient class
(connect, login, send, receive, disconnect) for iOS 15+ / macOS 13+.

Kotlin SDK: JNI bridge to libquicproquo_ffi with QpqClient class for
Android (aarch64, armv7) and JVM, Gradle build configuration.

Adds RegisterPushToken RPC (method ID 710) to device.proto for
APNs/FCM/WebPush device push token registration.

sdks/kotlin/jni/dev_quicproquo_NativeBridge.c

@@ -0,0 +1,117 @@
+/**
+ * JNI bridge between Kotlin/Java and libquicproquo_ffi.
+ *
+ * Compile with:
+ *   gcc -shared -fPIC -o libquicproquo_jni.so \
+ *       -I"$JAVA_HOME/include" -I"$JAVA_HOME/include/linux" \
+ *       dev_quicproquo_NativeBridge.c \
+ *       -L ../../../../target/release -lquicproquo_ffi
+ */
+
+#include <jni.h>
+#include <stdint.h>
+#include <string.h>
+
+/* Forward declarations for libquicproquo_ffi functions. */
+typedef struct QpqHandle QpqHandle;
+
+extern QpqHandle* qpq_connect(const char* server, const char* ca_cert, const char* server_name);
+extern int qpq_login(QpqHandle* handle, const char* username, const char* password);
+extern int qpq_send(QpqHandle* handle, const char* recipient,
+                     const uint8_t* message, size_t message_len);
+extern int qpq_receive(QpqHandle* handle, uint32_t timeout_ms, char** out_json);
+extern void qpq_disconnect(QpqHandle* handle);
+extern const char* qpq_last_error(const QpqHandle* handle);
+extern void qpq_free_string(char* ptr);
+
+/* --- JNI exports --- */
+
+JNIEXPORT jlong JNICALL
+Java_dev_quicproquo_NativeBridge_nativeConnect(
+    JNIEnv* env, jclass cls,
+    jstring server, jstring caCert, jstring serverName)
+{
+    const char* s = (*env)->GetStringUTFChars(env, server, NULL);
+    const char* c = (*env)->GetStringUTFChars(env, caCert, NULL);
+    const char* n = (*env)->GetStringUTFChars(env, serverName, NULL);
+
+    QpqHandle* h = qpq_connect(s, c, n);
+
+    (*env)->ReleaseStringUTFChars(env, server, s);
+    (*env)->ReleaseStringUTFChars(env, caCert, c);
+    (*env)->ReleaseStringUTFChars(env, serverName, n);
+
+    return (jlong)(uintptr_t)h;
+}
+
+JNIEXPORT jint JNICALL
+Java_dev_quicproquo_NativeBridge_nativeLogin(
+    JNIEnv* env, jclass cls,
+    jlong handle, jstring username, jstring password)
+{
+    QpqHandle* h = (QpqHandle*)(uintptr_t)handle;
+    const char* u = (*env)->GetStringUTFChars(env, username, NULL);
+    const char* p = (*env)->GetStringUTFChars(env, password, NULL);
+
+    int code = qpq_login(h, u, p);
+
+    (*env)->ReleaseStringUTFChars(env, username, u);
+    (*env)->ReleaseStringUTFChars(env, password, p);
+    return code;
+}
+
+JNIEXPORT jint JNICALL
+Java_dev_quicproquo_NativeBridge_nativeSend(
+    JNIEnv* env, jclass cls,
+    jlong handle, jstring recipient, jbyteArray message)
+{
+    QpqHandle* h = (QpqHandle*)(uintptr_t)handle;
+    const char* r = (*env)->GetStringUTFChars(env, recipient, NULL);
+    jsize len = (*env)->GetArrayLength(env, message);
+    jbyte* bytes = (*env)->GetByteArrayElements(env, message, NULL);
+
+    int code = qpq_send(h, r, (const uint8_t*)bytes, (size_t)len);
+
+    (*env)->ReleaseByteArrayElements(env, message, bytes, JNI_ABORT);
+    (*env)->ReleaseStringUTFChars(env, recipient, r);
+    return code;
+}
+
+JNIEXPORT jstring JNICALL
+Java_dev_quicproquo_NativeBridge_nativeReceive(
+    JNIEnv* env, jclass cls,
+    jlong handle, jint timeoutMs)
+{
+    QpqHandle* h = (QpqHandle*)(uintptr_t)handle;
+    char* json = NULL;
+    int code = qpq_receive(h, (uint32_t)timeoutMs, &json);
+
+    if (code != 0 || json == NULL) {
+        if (json) qpq_free_string(json);
+        return NULL;
+    }
+
+    jstring result = (*env)->NewStringUTF(env, json);
+    qpq_free_string(json);
+    return result;
+}
+
+JNIEXPORT void JNICALL
+Java_dev_quicproquo_NativeBridge_nativeDisconnect(
+    JNIEnv* env, jclass cls, jlong handle)
+{
+    QpqHandle* h = (QpqHandle*)(uintptr_t)handle;
+    if (h) {
+        qpq_disconnect(h);
+    }
+}
+
+JNIEXPORT jstring JNICALL
+Java_dev_quicproquo_NativeBridge_nativeLastError(
+    JNIEnv* env, jclass cls, jlong handle)
+{
+    QpqHandle* h = (QpqHandle*)(uintptr_t)handle;
+    const char* err = qpq_last_error(h);
+    if (!err) return NULL;
+    return (*env)->NewStringUTF(env, err);
+}