linux-cachyos 7.2.4-1: RX 7900 XTX disappears from PCIe after Plasma Wayland login

## Affected

- Kernel: `linux-cachyos 7.2.4-1`
- GPU: Sapphire Radeon RX 7900 XTX (Navi 31, PCI ID `1002:744c`)
- Driver: `amdgpu`
- Desktop: Plasma Wayland / KWin
- Mesa: `26.2.2-2`

## Known-good comparison

- `linux-cachyos-lts 6.18.50-1` works normally on the same machine.
- I have not yet tested Arch's vanilla `linux` kernel.

## Symptoms

After entering my password in Plasma Login Manager, the session freezes within seconds. The screen goes black and the system may restart itself. Input becomes unresponsive.

## Reproduction

1. Boot `linux-cachyos 7.2.4-1`.
2. Log into Plasma Wayland.
3. Within a few seconds, the display freezes/blacks out and the system crashes or restarts.

## Troubleshooting performed

- Disabled `lactd.service` and rebooted.
- Confirmed LACT did not run during the failing boot.
- The failure still reproduced.
- Booting `linux-cachyos-lts 6.18.50-1` resolves the issue.

## Relevant journal lines

```text
amdgpu 0000:03:00.0: Dumping IP State
amdgpu 0000:03:00.0: device lost from bus!
amdgpu 0000:03:00.0: MES failed to respond to msg=MISC (WAIT_REG_MEM)
amdgpu 0000:03:00.0: failed to reg_write_reg_wait
kwin_wayland: Pageflip timed out! This is a bug in the amdgpu kernel driver
amdgpu 0000:03:00.0: [drm] *ERROR* [CRTC:376:crtc-0] flip_done timed out
```

## Diagnostic bundle

https://paste.cachyos.org/p/44f41a3.log

Start by reviewing the diagnostic bundle and the listed amdgpu, MES, and KWin journal errors, then reproduce the failure with linux-cachyos 7.2.4-1 after Plasma Wayland login. Compare that behavior with the known-good linux-cachyos-lts 6.18.50-1 and, if available, Arch's vanilla linux. Done means identifying and resolving the kernel or driver regression causing the GPU to disappear from PCIe.

Written by the indexing model from the issue text.