[Bug]: Display scaling changes stop working after upgrade from Plasma 6.6.5 → 6.7.0.

### Kernel

7.0.12-1-cachyos

### Desktop Environment / Window Manager

KDE Plasma

### CPU

AMD Ryzen 7 5800X3D

### GPU / Driver

AMD Radeon RX 9070 XT/Mesa 26.1.3-arch2.1

### Description

Upgrade from Plasma 6.6.5 → 6.7.0.
Display scaling changes stop working.
GUI reports "Implementation error".
kscreen-doctor reports "Cannot find output with id 1".
Outputs are still listed correctly by kscreen-doctor -o.

### Steps to Reproduce

1. Setting scaling via System Settings/Display Configuration
2. kscreen-doctor output.DP-4.scale.1

### Expected Behavior

The screen to change scaling and for to accept changes.

### Actual Behavior

Nothing changes on the displays, in gui if you press apply it comes up with the do you want to keep or revet but nothing actually changes ans if you go out of the config screen it asks you apply but again not5hing changes.

### Logs / Error Messages

https://paste.cachyos.org/p/6ab797e

### Additional Context

_No response_

Start by reproducing the scaling failure after upgrading from Plasma 6.6.5 to 6.7.0, using the reported System Settings path and `kscreen-doctor output.DP-4.scale.1` command. Compare `kscreen-doctor -o` output with the linked log and determine why output id 1 cannot be found; done means scaling changes apply successfully through both interfaces.

Written by the indexing model from the issue text.