feat: `designlang timeline <url>` — track design evolution

[Manavarya09]

Summary

`history.js` saves snapshots. Add `timeline` to render them as a visual story.

```bash
designlang timeline stripe.com --since 2024

→ stripe-com.timeline.html — visual changelog of palette, type, spacing across snapshots

```

Why

"How did Stripe's design evolve in 2025?" is a content magnet. Every site with history snapshots becomes a shareable evolution page.

Approach

• Read snapshots from `~/.designlang/history`
• Diff each pair (existing diffDesigns)
• Emit a vertical timeline HTML — each row = a snapshot, with palette delta + type changes + grade arc

Scope

~4h.