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/swift/Sources/CQuicProQuo/module.modulemap

@@ -0,0 +1,5 @@
+module CQuicProQuo {
+    header "quicproquo_ffi.h"
+    link "quicproquo_ffi"
+    export *
+}

sdks/swift/Sources/CQuicProQuo/quicproquo_ffi.h

@@ -0,0 +1,52 @@
+// quicproquo_ffi.h — C header for libquicproquo_ffi.
+//
+// Mirrors the extern "C" functions from crates/quicproquo-ffi/src/lib.rs.
+
+#ifndef QUICPROQUO_FFI_H
+#define QUICPROQUO_FFI_H
+
+#include <stddef.h>
+#include <stdint.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+// Status codes.
+#define QPQ_OK             0
+#define QPQ_ERROR          1
+#define QPQ_AUTH_FAILED    2
+#define QPQ_TIMEOUT        3
+#define QPQ_NOT_CONNECTED  4
+
+// Opaque handle type.
+typedef struct QpqHandle QpqHandle;
+
+// Connect to a quicproquo server.  Returns NULL on failure.
+QpqHandle* qpq_connect(const char* server, const char* ca_cert, const char* server_name);
+
+// Authenticate with OPAQUE credentials.
+int qpq_login(QpqHandle* handle, const char* username, const char* password);
+
+// Send a message to a recipient (by username).
+int qpq_send(QpqHandle* handle, const char* recipient,
+             const uint8_t* message, size_t message_len);
+
+// Receive pending messages (blocking).  On success, *out_json is a
+// heap-allocated JSON string that must be freed with qpq_free_string.
+int qpq_receive(QpqHandle* handle, uint32_t timeout_ms, char** out_json);
+
+// Disconnect and free the handle.
+void qpq_disconnect(QpqHandle* handle);
+
+// Return the last error message, or NULL.  Do NOT free the result.
+const char* qpq_last_error(const QpqHandle* handle);
+
+// Free a string returned by qpq_receive.
+void qpq_free_string(char* ptr);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif // QUICPROQUO_FFI_H