feat: async C API and Swift radio state display

Split the dongle session into a fast probe (xone_open) and a background
start (xone_start) that loads the firmware and initializes the radio on
a worker thread, so the app UI never blocks. Add xone_state/xone_error
for progress display, and select the firmware image per product ID when
no path is given. The Swift app polls the state each second and shows
idle/starting/ready/error plus the firmware build or error string.

The session destructor joins the worker so quitting the app does not
terminate on a joinable thread. Note in the CLI and transport that the
recover/re-enumerate path should not be used yet: a crashed SIE drops
off the bus, while the MCU watchdog recovers it after ~90s of quiet.

Co-Authored-By: qwen3.8-27b@q2_k_xl: async C API, Swift state display, and exit-time worker join

src/usb/usb_transport.cpp

@@ -170,6 +170,9 @@ void transport::stop_pump()
 
 auto transport::re_enumerate() -> int
 {
+    // Do not use yet: a crashed SIE cannot finish the host-side port reset
+    // handshake and drops off the bus. A crashed chip recovers on its own via
+    // the MCU watchdog after ~90s of a quiet bus, so prefer waiting.
     // Stop the pump first so no completion callback touches the interface
     // references while the kernel tears them down.
     stop_pump();