Add automated production Core Web Vitals monitoring

Problem

The repository enforces generated asset-size budgets, but docs/SYSTEM_STATE.md notes there is no automated production Core Web Vitals trace. Build-time size limits do not show how the deployed site performs under real browser/network conditions.

Goal

Measure the deployed site's user-visible performance with a lightweight, reproducible production check.

Scope

• Select a small set of representative public routes, including the homepage and one long-form page.
• Measure Core Web Vitals or the closest deterministic lab equivalents against production.
• Record baselines and reasonable regression thresholds.
• Keep the mechanism low-maintenance and avoid adding visitor-side analytics solely for this purpose.
• Integrate with release confidence where the signal is stable enough.

Acceptance criteria

• Production performance is measured automatically for representative routes.
• Baselines and regression thresholds are documented.
• Results distinguish deploy/network noise from meaningful regressions where practical.
• No unnecessary third-party tracking is added to visitors.
• Release documentation points to the performance check.

Start with docs/SYSTEM_STATE.md and identify the existing production routes, generated asset-budget checks, and release documentation. Define a reproducible production measurement for the homepage and a long-form page, document baselines and thresholds, and finish when the automated check and release guidance satisfy all listed acceptance criteria.