[feat] add native iOS client support

ios/README.md

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+# CrossDesk Mobile (native iOS)
+
+This target is a native MiniRTC controller. It uses the same WebSocket
+signaling, libnice ICE, SRTP/RTP and data-stream protocol as the desktop app.
+There is no `WKWebView` or browser runtime.
+
+## Requirements
+
+- Xcode 16 or newer
+- Xmake available on `PATH`
+- A physical arm64 iPhone or iPad running iOS 16 or newer
+
+## Build
+
+Open `CrossDeskMobile.xcodeproj`, select the `CrossDeskMobile` scheme and a
+physical device, then build. The first build phase compiles MiniRTC and the
+shared CrossDesk protocol, then merges their iPhoneOS static dependencies into
+a local archive under `Vendor/`.
+
+You can also verify the target without code signing:
+
+```sh
+xcodebuild -project ios/CrossDeskMobile.xcodeproj \
+  -scheme CrossDeskMobile \
+  -configuration Debug \
+  -destination 'generic/platform=iOS' \
+  CODE_SIGNING_ALLOWED=NO build
+```
+
+The first connection to a signaling server provisions and stores an identity
+for that server. Remote control sessions then log in as `C-<identity>` and use
+the desktop-compatible `DisplayN`, `control_audio`, `mouse`, `keyboard`,
+`control_data`, `clipboard`, `file`, and `file_feedback` streams.
+
+## Video codecs
+
+Every iOS build includes VideoToolbox, OpenH264, dav1d, libaom, and SVT-AV1. H.264 uses
+VideoToolbox by default; the in-app **Settings > Video Codec** selector switches
+H.264 software processing to OpenH264. AV1 encoding uses SVT-AV1 and AV1 decoding
+uses dav1d; VideoToolbox AV1 hardware decoding is currently unsupported.
+
+## Physical-device test checklist
+
+Run the app from Xcode on a physical device and connect to a current desktop
+build. Test the features in this order so a media problem is not confused with
+a data-channel problem:
+
+1. **Video:** after the connection turns green, the waiting panel should be
+   replaced by the remote desktop. The status bar reports the decoded size and
+   frame count.
+2. **Audio:** play continuous sound on the remote computer, then toggle the
+   speaker button. Audio is Opus-decoded by MiniRTC and played as 48 kHz mono
+   16-bit PCM through `AVAudioEngine`.
+3. **Clipboard:** copy a short text value on the phone and choose **Send local
+   clipboard**. Copy a different value on the desktop and confirm that it is
+   written to the iOS pasteboard. Text is limited to 128 KiB.
+4. **Displays:** open the display menu, switch every listed monitor, and check
+   that the frame counter restarts and the selected monitor appears after a
+   new key frame.
+5. **Files:** use the folder button to pick a small file, then send a file back
+   from the desktop. Progress is ACK-driven. Received files are stored in the
+   app's `Documents/Received` directory and can be exported with the share
+   button or Finder's Files tab.
+
+For black-screen diagnosis, keep Xcode's device console open and filter for
+`CrossDesk`. A working pipeline prints both `VideoToolbox decoded frame` from
+MiniRTC and `CrossDesk received decoded frame` from the iOS bridge. If only the
+first message appears, verify that the selected stream is named `Display1`,
+`Display2`, and so on. If neither appears, update and rebuild the desktop side
+and request a key frame.