feat(theme): add the page-enter and content-enter arrival animations

[gabriel-lisboa-azion]

Summary

• animate-page-enter — the route transition. Starts one --layout-boundary-inline to the left of where it lands and travels right while fading up, so a page reads as arriving from the nav row that was clicked; a plain fade has no origin and a right-side arrival reads as going back. Retune per shell with --page-enter-distance.
• animate-content-enter — content settling inside a page that is already on screen. Rises a hair instead of travelling, and takes a --content-enter-delay stagger (backwards fill is load-bearing: without it a delayed follower paints its landed state and then jumps back).
• Neither fills forwards, deliberately — page-enter's box is usually the scroll container, and a lingering translate would make it a containing block for any position: fixed descendant.
• webkit-motion-polish documents both, plus the failure they exist to prevent: a page arrives once, so a mount-time entrance inside a page whose route transition is still running lock-steps with it into one diagonal move.

How to test

1. node packages/theme/src/scripts/build-tokens.mjs → packages/theme/dist/v4/globals.{css,scss} diff is exactly the two --animate-* entries (light + dark) and the two @keyframes; nothing else moves.
2. Probe that the utilities actually emit CSS (they are dead-silent if they do not — see .claude/rules/styling.md), through the v4 compile API with a bogus class as the control:

   compile(globals.css).build(['animate-page-enter','animate-content-enter','totally-bogus-control-class'])
   

   → the first two emit animation: var(--animate-page-enter|content-enter), the control emits nothing.
3. node packages/webkit/scripts/build-catalog.mjs → tokens.animations is 21 and contains both.
4. cd packages/webkit && node --test "test/**/*.test.mjs" → 115 pass.

Notes

• Timing is moderate-02 + productive-entrance for both — the pairing the system already uses for a panel of content arriving, so a page and the components on it move as one system.
• catalog.json is rebuilt in the same PR: until it carries the two names, validate-spec-compliance rejects animate-page-enter in a component.