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/app/xone_api.h

@@ -8,6 +8,7 @@
 // ==============================================================================
 
 #include <stdbool.h>
+#include <stdint.h>
 
 #ifdef __cplusplus
 extern "C" {
@@ -19,6 +20,32 @@ const char *xone_version(void);
 // TRUE if an Xbox Wireless Dongle is connected, FALSE otherwise.
 bool xone_dongle_present(void);
 
+// Opaque handle to an open dongle session (probe + firmware load).
+// Create with xone_open(); call the getters from the thread that opened
+// it, and before xone_close(). Only one session may be open at a time.
+typedef struct xone_dongle xone_dongle;
+
+// Open a dongle session: probe, USB reset, and (if firmware_path is not
+// NULL) load the firmware image from that file. Returns NULL if no dongle
+// is present or opening fails.
+xone_dongle *xone_open(const char *firmware_path);
+
+// Close the session and release the dongle.
+void xone_close(xone_dongle *d);
+
+// Product ID of the connected dongle (e.g. 0x02E6).
+uint16_t xone_pid(const xone_dongle *d);
+
+// Chip ID from EFUSE (e.g. 0x7612 for MT7612). 0 if unknown.
+uint16_t xone_chip_id(const xone_dongle *d);
+
+// MAC address from EFUSE as "xx:xx:xx:xx:xx:xx". Valid until xone_close().
+const char *xone_mac_address(const xone_dongle *d);
+
+// Build string of the loaded firmware image (e.g. "201703281033____").
+// Empty string if the firmware was not loaded. Valid until xone_close().
+const char *xone_firmware_build(const xone_dongle *d);
+
 #ifdef __cplusplus
 }
 #endif

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