test(production): benchmark and document sidecar benefit

[zackees]

Parent: #471
Depends on #475 and #476

Goal

Document and measure the conditions under which the optional sidecar has a real remote-execution or peak-memory benefit over direct Playwright transport.

Acceptance criteria

• Benchmark compares direct and sidecar memory/remote execution behavior using a reproducible workload.
• Documentation explains transport selection, loopback/private deployment boundaries, and retained static GitHub Pages compatibility.