Implement Telemetry Probe for Worker Saturation Monitoring

To diagnose potential worker thread saturation (which causes scroll lag but is invisible to static analysis), we need a runtime telemetry tool.

Goal: Implement a "Heartbeat" probe to measure the Round-Trip Time (RTT) across the worker triangle.

Architecture:

1. Main Thread Addon: Sends a timestamped "ping" message.
2. App Worker: Relays the ping to VDOM Worker.
3. VDOM Worker: Relays the ping back to Main Thread.
4. Main Thread: Calculates RTT = Now - OriginalTimestamp.

Usage:

• Low RTT (<16ms): Healthy.
• High RTT (>50ms): Saturation (VDOM diffing, GC, or Serialization bottleneck).

This provides a definitive "Lag Meter" to identify if performance issues are computational or rendering-based.