Verify Desktop telemetry captures exceptions, unhandled rejections, and reportError

## Problem

Desktop telemetry is being considered as an ECS canary signal, but coverage for generalized exceptions, unhandled promise rejections, and explicit reportError calls has not been proven. A missing capture path would turn absence of reports into false reassurance.

## Goal

Prove which frontend failures reach Desktop telemetry before using it for ECS soak decisions.

## Proposed Solution

Run controlled positive probes for each error path in a Desktop canary build, verify receipt and version/build attribution, and document any blind spots.

## Acceptance Criteria

- Controlled probes cover thrown exceptions, unhandled rejections, and reportError.
- Each signal is observed end to end with frontend/Desktop version attribution.
- Missing paths have follow-up fixes and detectors.
- ECS soak guidance states exactly what Desktop telemetry can and cannot detect.