owner-led(visual-agent): visual diff + change quantification

[JSONbored]

Parent roadmap: #525
Parent epic: #577

Owner-led — assigned to @JSONbored.

Goal

Visual diff + change quantification: compare before/after per route, produce a diff image and a changed-pixel percentage, and classify "changed / unchanged / new / removed".

Reuse

pixelmatch (or Playwright's toHaveScreenshot snapshot diff).

Scope

• Pixel diff + diff-image generation; anti-alias tolerance.
• Per-route change % + an overall summary; threshold to suppress noise.

Acceptance

Routes with real visual changes are flagged with a diff image + change %; unchanged routes are marked unchanged.