Warm-cache second-visit metrics (cache-effectiveness evidence)

[mgifford]

Size: S. In scanRoute, optionally reload the page in the same context and record a second PageMetrics (warmMetrics). The cold/warm delta per page measures cache-header effectiveness — hard evidence for the WSG alignment statement's caching claims.

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Scoped in TOOLING-ROADMAP.md — sized to complete in one AI session. Drafted with AI assistance (Claude Code); no external code copied.