Investigate 350-726 ms native renderer stalls remaining after ECS redraw-loop isolation

## Problem

The reporter trace analyzed for #15977 contains CPU-active native/unattributed renderer stalls of 350-726 ms. The controlled headless harness proved the ECS redraw feedback loop but did not reproduce or explain these stalls. Treating the redraw fix as a complete explanation would drop a distinct performance signal.

## Goal

Attribute or bound the residual native stalls under a comparable headed production environment.

## Proposed Solution

Capture native stacks and a headed production trace on comparable OS, GPU, refresh rate, workflow execution, and DevTools settings. Compare with and without Memory capture and 3D preview.

## Acceptance Criteria

- The environment and trace provenance are versioned and reproducible.
- Native stacks or equivalent attribution are captured for the longest tasks.
- Host contention, browser/GPU work, 3D preview, and profiler perturbation are separately tested.
- Confirmed causes receive bounded fix issues; ruled-out causes are recorded.