[BUG] Touchpad not detected on linux-cachyos 7.1.1 (regression from 7.0.12) — ThinkPad Z13 Gen 1, works on 6.18-LTS

## Summary

After updating `linux-cachyos` from **7.0.12-1 → 7.1.1-2**, the touchpad is no longer detected at all. Only the TrackPoint enumerates. Booting `linux-cachyos-lts` **6.18.36-1** (same system, same `linux-firmware`) fully restores the touchpad, so this is a kernel regression in the 7.1.x series, not a firmware/userspace issue.

## Hardware

- **Model:** Lenovo ThinkPad Z13 Gen 1 (21D2CTO1WW)
- **CPU:** AMD Ryzen 7 PRO 6860Z
- **BIOS:** N3GET79W (1.79), 2026-04-28
- **Pointing devices:** Elan TrackPoint + touchpad, behind the i8042/PS/2 AUX port; Wacom touchscreen (unaffected)

## Versions

| | Version | Touchpad |
|---|---|---|
| Bad | `linux-cachyos` 7.1.1-2 | **not detected** |
| Good | `linux-cachyos` 7.0.12-1 (previous) | works |
| Good | `linux-cachyos-lts` 6.18.36-1 | works |

`linux-firmware` / `amd-ucode` are identical across the good and bad boots (both at `20260622`), ruling out firmware.

## Symptoms / evidence (on 7.1.1-2)

`/proc/bus/input/devices` contains **no touchpad node** — only `TPPS/2 Elan TrackPoint`, the keyboard, and the Wacom touchscreen.

Kernel log shows psmouse registering *only* the TrackPoint on serio1, with **no Synaptics/touchpad detection line** preceding it and no RMI/SMBus handoff:

```
psmouse serio1: trackpoint: Elan TrackPoint firmware: 0x00, buttons: 3/3
input: TPPS/2 Elan TrackPoint as /devices/platform/i8042/serio1/input/input8
```

On 6.18.36-LTS the touchpad enumerates normally on the same boot path.

## What was already tried (no effect on 7.1.1)

- `modprobe -r psmouse && modprobe psmouse` — re-probes serio1, still only finds the TrackPoint
- `modprobe psmouse synaptics_intertouch=1` — no change
- Confirmed no modprobe blacklist, `synaptics_intertouch=-1` (auto), keyboard layout normal
- Touchpad is not appearing on any I2C-HID or SMBus/RMI4 address either

## Suspected area

i8042 / psmouse touchpad (Synaptics/RMI) detection regressed somewhere between 7.0.12 and 7.1.1. Similar in spirit to #858.

## Happy to help

I can provide full `dmesg`, run with `psmouse.*` debug params, or do a `git bisect` between 7.0.12 and 7.1.1 if that would help narrow it down.

Start by comparing the i8042/psmouse behavior and dmesg output between linux-cachyos 7.0.12-1 and 7.1.1-2, using the reported psmouse debug parameters if needed. A git bisect between those versions may narrow the regression; done means the ThinkPad Z13 touchpad is detected again on 7.1.x without breaking the TrackPoint.

Written by the indexing model from the issue text.