Hotplug: device->next must be NULL in callback #791

Tracking from #790 / #674.

Contract (documented in #790)

The device->next pointer is always NULL. Each callback invocation describes exactly one device; compound or composite devices that expose multiple interfaces produce multiple callback invocations (typically delivered in quick succession).

Current state

Backends invoke the user callback with a hid_device_info* whose next field still points into the backend-internal enumeration list. Example: libusb/hid.c:1123 already carries a `TODO` for this:

```c
/* TODO: possibly make the `next` field NULL to match the behavior on other systems */
```

The other backends are similarly inconsistent.

Proposed fix

Before dispatching the callback, copy the `hid_device_info` into a local struct on the dispatcher's stack with `.next = NULL`, then pass a pointer to that local struct into the callback. Do not deep-copy the string fields — they are still owned by HIDAPI and remain valid for the duration of the callback.

This avoids exposing the internal list layout to user code and gives one consistent cross-backend contract.

Backends to touch

• `libusb/hid.c` — `hid_internal_invoke_callbacks`
• `linux/hid.c`
• `mac/hid.c`
• `windows/hid.c`

Related

• RFC: Hotplug implementation #674 (hotplug implementation)
• A bit of documentation details on callback API #784 (original doc patch, superseded)
• Hotplug docs: clarify thread-safety, safe-to-call list, device->next contract #790 (documentation of the target contract)