v165 - single metro / 125ms stagger test
Base: v163, confirmed non-flashing and responsive.

Removed the v164 extra 125ms metro approach that froze UI/page switching.
There is still only ONE continuous metro: 250ms.

Each 250ms tick now schedules:
- A frame immediately
- B frame 125ms later

Goal: visible 125ms shimmer while avoiding a second continuously-running metro.
Exact old preferred A/B geometry retained.
Audio/AUX/LED untouched.
