feat(transport): implement macOS USB discovery using IOKit

[skot]

Summary

• add macOS USB discovery using IOKit
• support device enumeration and hotplug notifications
• map USB devices to /dev/cu.* serial ports by location ID
• document macOS support in the README

Validation

• cargo test --no-run

[skot]

tested with a bitaxe-raw bitaxeGamma from MacOS and seems to be working.

[majoal0]

Tested on macOS with a BitAxe Gamma 602 connected via USB-C. The device was detected correctly and communication through Mujina worked as expected.

[rkuester]

I assume you needed cargo test --no-run because cargo test hung. That's the bug tracked in #46. It should be fixed now by 9ea2497. Please try running cargo test again and let me know if it works for you on macOS.

[Nickamoto]

Tested on: Mac mini (Apple M-series, macOS 26.2) with a Bitaxe Gamma (BM1370, AxeOS v2.13.0) connected via USB.

Build: Clean. No new errors or warnings introduced by this PR.

Hardware verification: The Bitaxe enumerates in IORegistry under IOUSBHostDevice (VID 0x303A / Espressif) with an associated IOSerialBSDClient at /dev/cu.usbmodem*. This is exactly the class and serial port path the PR targets.

Issues found:

1. Race condition in monitor_blocking — enumerate_devices() is called before IOServiceAddMatchingNotification is registered. A device plugged in during that window will be missed. The standard IOKit pattern avoids this: register the notification first, then drain the returned iterator — the drain step already yields existing devices. The separate enumerate_devices() call is both redundant and racy.

2. kIOMasterPortDefault is deprecated since macOS 12 — kIOMainPortDefault is the current replacement. Functionally equivalent (both are 0), but worth updating to avoid future warnings.

3. Stale module doc in usb.rs — Line 10 still reads "macOS: Stub implementation (IOKit support planned for future)". Should be updated to reflect that macOS is now fully supported.

4. _master_port field in MacOsIoKitDiscovery - Stores kIOMasterPortDefault (the constant 0) but is never used. The field could be removed entirely.

After flashing bitaxe-raw (esp-hal-update branch) onto the Bitaxe, re-ran the same discovery tests. The firmware presents a different USB descriptor than AxeOS:
• bitaxe-raw: VID=c0de PID=cafe, product="Bitaxe", manufacturer="OSMU"
• AxeOS: VID=303a PID=1001, product="USB JTAG/serial debug unit", manufacturer="Espressif"
Both are correctly enumerated by MacOsIoKitDiscovery. Under bitaxe-raw, two serial ports are exposed and both are correctly resolved by find_serial_ports_for_device:
• /dev/cu.usbmodem9e79bedc1 — control serial (I2C, GPIO, ADC)
• /dev/cu.usbmodem9e79bedc3 — data serial (ASIC UART passthrough)