Add traffic stats callback to FFI (iOS + Android)

[AYastrebov]

Summary

• Wire cumulative upload/download byte counters through the TUN server via a new src/tun/traffic.rs module
• TCP bytes counted after copy_bidirectional completes (both directions)
• UDP bytes counted per-message in destination tasks (download on read, upload on write)
• 1-second tokio::time::interval timer in run_tun_server invokes the platform callback with current totals
• iOS: shoes_start now takes a ShoesTrafficCallback C function pointer parameter
• Android: ShoesNative.start now takes a TrafficListener interface parameter

Architecture

src/tun/traffic.rs provides:

• Two AtomicU64 globals (UPLOAD_BYTES / DOWNLOAD_BYTES)
• RwLock<Option<TrafficCallback>> for the platform callback
• Helpers: add_upload_bytes, add_download_bytes, reset_traffic_counters, set/clear_traffic_callback, report_traffic

Platform notes

• iOS: C function pointers can't capture context — the Swift side uses a private weak var activeProvider global as an indirect capture; the closure reads it at call time
• Android: TrafficListener stored as GlobalRef<JObject>, JVM attached per call via attach_current_thread

Files changed

File	Change
src/tun/traffic.rs	New module — counters, callback, tests
src/tun/mod.rs	1s reporting timer in TUN event loop
src/tun/udp_manager.rs	Per-message byte counting
src/ffi/ios.rs	ShoesTrafficCallback parameter
src/ffi/android.rs	TrafficListener JNI integration
include/shoes.h	C header updated
ShoesNative.kt	Kotlin interface + JNI bridge

Test plan

• cargo test --lib tun::traffic — 4 unit tests pass (counters, callback, clear, no-panic)
• iOS: verify ShoesTrafficCallback is invoked every ~1s with increasing byte counts
• Android: verify TrafficListener.onTrafficUpdate fires with correct totals

🤖 Generated with Claude Code

[cfal]

one downside to this is that it waits for copy_bidirectional to finish - which means if it's a large download, you won't see it immediately and it won't be useful if we ever want to display speeds. we probably need some wiring inside of copy_bidirectional to do better.

[AYastrebov]

Good point! Fixed — replaced the post-hoc counting with a TrafficCountingStream wrapper that intercepts poll_read/poll_write on the local TUN connection. Bytes are now reported to the atomic counters in real time as they flow through copy_bidirectional, so long-lived connections show traffic immediately and this would support speed display in the future.

The TCP counting lines in mod.rs are replaced with:

let mut counting = traffic::TrafficCountingStream::new(connection);
let result = tokio::io::copy_bidirectional(&mut counting, &mut remote).await;

UDP counting remains per-message (already incremental).