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/README.md

@@ -0,0 +1,104 @@
+# QuicProQuo Swift SDK
+
+Swift wrapper over `libquicproquo_ffi` for iOS and macOS.
+
+## Prerequisites
+
+- Xcode 15+ / Swift 5.9+
+- iOS 15+ or macOS 13+
+- `libquicproquo_ffi` built for the target architecture
+
+## Building the Native Library
+
+```sh
+# iOS (device)
+cargo build --release -p quicproquo-ffi --target aarch64-apple-ios
+
+# iOS Simulator (Apple Silicon)
+cargo build --release -p quicproquo-ffi --target aarch64-apple-ios-sim
+
+# macOS
+cargo build --release -p quicproquo-ffi --target aarch64-apple-darwin
+```
+
+## Installation
+
+### Swift Package Manager
+
+Add to your `Package.swift`:
+
+```swift
+dependencies: [
+    .package(path: "sdks/swift"),
+]
+```
+
+Or add the library search path to your Xcode project:
+
+```
+LIBRARY_SEARCH_PATHS = $(PROJECT_DIR)/../target/release
+OTHER_LDFLAGS = -lquicproquo_ffi
+```
+
+## Usage
+
+```swift
+import QuicProQuo
+
+let client = try QpqClient(
+    server: "127.0.0.1:5001",
+    caCertPath: Bundle.main.path(forResource: "ca", ofType: "pem")!
+)
+
+try client.login(username: "alice", password: "secret")
+try client.send(to: "bob", message: "hello".data(using: .utf8)!)
+
+let messages = try client.receive(timeoutMs: 5000)
+for msg in messages {
+    print("Received: \(msg)")
+}
+
+client.disconnect()
+```
+
+## API
+
+| Method | Description |
+|---|---|
+| `QpqClient(server:caCertPath:serverName:)` | Connect to server |
+| `client.login(username:password:)` | OPAQUE authentication |
+| `client.send(to:message:)` | Send message by username |
+| `client.receive(timeoutMs:)` | Receive pending messages |
+| `client.disconnect()` | Disconnect |
+| `client.isConnected` | Connection status |
+
+## Error Handling
+
+All errors are thrown as `QpqError`:
+
+```swift
+do {
+    try client.login(username: "alice", password: "wrong")
+} catch QpqError.authFailed(let msg) {
+    print("Authentication failed: \(msg)")
+} catch QpqError.connectionFailed(let msg) {
+    print("Connection failed: \(msg)")
+}
+```
+
+## Structure
+
+- `Sources/CQuicProQuo/` -- C module map and FFI header
+- `Sources/QuicProQuo/QpqClient.swift` -- High-level Swift client
+- `Sources/QuicProQuo/QpqError.swift` -- Error types
+
+## Cross-Compilation
+
+For iOS devices, add the Rust target and build:
+
+```sh
+rustup target add aarch64-apple-ios
+cargo build --release -p quicproquo-ffi --target aarch64-apple-ios
+```
+
+Copy the library to your Xcode project's framework search path.