Power Display: Audio volume control via DDC/CI (VCP 0x62)

### Description of the new feature / enhancement

Add support for reading and setting the audio volume of monitors that expose VCP feature code 0x62 via DDC/CI, allowing control of built-in monitor speakers directly from Power Display.

### Scenario when this would be used?

Windows already exposes monitor speakers as an HDMI audio endpoint, but adjusting volume through Windows only acts at the OS level: it does not send a DDC/CI command to the monitor itself.

`VCP 0x62` sets the physical volume on the monitor, which matters when the monitor receives audio from multiple HDMI sources beyond the PC.

Since Power Display already uses DDC/CI for input source switching, color profiles and rotation, this would require no new protocol work. Implementation effort appears low relative to the feature parity gained with ClickMonitorDDC.

### Supporting information

- VCP code `0x62` (Audio Speaker Volume) is defined in the MCCS standard
- ClickMonitorDDC supports monitor audio volume control

Start by locating Power Display's existing DDC/CI controls for input source switching, color profiles, or rotation. Add read and set behavior for MCCS VCP feature 0x62, then verify that monitor volume can be retrieved and changed through the existing Power Display controls.

Written by the indexing model from the issue text.