feat: expose dongle debug info through the C API

Add an opaque xone_dongle session handle to the C ABI bridge:
xone_open() probes, resets, and loads the firmware (path optional),
and xone_pid/xone_chip_id/xone_mac_address/xone_firmware_build return
debug info for the open session. chip captures the firmware build
string from the image header during load.

The Swift app opens a session when the dongle appears and shows PID,
chip ID, MAC address, and firmware build string in a debug section.

Co-Authored-By: qwen3.8-27b@q2_k_xl: added C API session handle and debug UI

include/mt76/mt76.hpp

@@ -45,6 +45,9 @@ public:
     // Resets the MCU if firmware is already loaded. Returns 0 or -errno.
     auto load_firmware(char const *path) -> int;
 
+    // Build string of the loaded firmware image (empty until load_firmware).
+    auto firmware_build() const -> char const *;
+
 private:
     // Send an MCU command (port of xone_mt76_send_command). `cmd` is the
     // MT_MCU_MSG_CMD_TYPE field. Returns bytes written, or a negative errno.
@@ -59,6 +62,7 @@ private:
     auto reset_firmware() -> int;
 
     usb::transport &transport_;
+    char build_time_[17] = {};
 };
 
 } // namespace xone::mt76