Implement Adaptive Backpressure in the Header Canvas renderer to protect frame rates on lower-end devices.

Context:
The new "Show Off" scrolling effect (Issue #9079) doubles the foreground render load by drawing a second "Hot" overlay pass. While efficient, this could cause frame drops on constrained hardware.

Requirements:

1. Frame Timing: Implement avgFrameTime tracking in the Header.mjs render loop (similar to Sparkline.mjs).
2. Stress Detection: Define a stressLevel metric.
3. Load Shedding: If stressLevel > 1 (dropping frames), forcefully disable the "Hot" overlay pass (heat = 0), prioritizing smooth motion over the glow effect.