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

include/usb/usb_transport.hpp

@@ -78,6 +78,11 @@ public:
     // rescan). Host-side operation that works even when EP0 is wedged. Any
     // existing transport handles are invalidated; probe() again for a fresh
     // session. Returns 0 on success, -errno on failure.
+    //
+    // Do not use yet: when the chip's SIE is crashed it cannot finish the
+    // host-side port reset handshake and the device drops off the bus
+    // entirely. A crashed chip recovers on its own via the MCU watchdog after
+    // ~90s of a quiet bus, so prefer waiting over re-enumeration.
     auto re_enumerate() -> int;
 
     ~transport();