feat: emit enterFrom classes from SSR when intro is set

[jonlaing]

Summary

Fix the FOUC on `intro: true`. Previously:

1. SSR renders intro-animated content in its final visual state.
2. Browser paints — user sees final state.
3. Hydration runs, applies `enterFrom` classes (e.g. `opacity-0`), user sees content disappear.
4. Enter animation transitions from `enterFrom` → final. User sees content fade back in.

The flash between steps 2 and 3 is the FOUC.

Now:

1. SSR renders content with `enterFrom` classes already applied — browser paints the pre-animation state (invisible).
2. Hydration's `runEnterAnimation` removes `enterFrom`, transitions to final state.

Same lifecycle as pure client-side rendering, no flash.

Plumbing

• `SSRControlCtx.addSlot` reads `AnimationConfigCtx` after rendering each slot.
• If `config.intro === true`, it extracts `enterFrom` from `config.list ?? config.single` and calls `renderer.toggleClass(element, cls, true)` for each class name.
• Uses the renderer's own class primitive (not `Element.addClass`, which relies on DOM `classList`) so it works with the SSR StringRenderer's VNode model.
• Only fires when `intro: true` — ordinary SSR output is unchanged.

Test plan

• `npx tsc --noEmit` clean
• Three new tests in `server.test.ts`:
  • `each({ intro: true })` → SSR HTML contains `enterFrom` classes on every item
  • `each({ ... })` without intro → SSR HTML has no `enterFrom` classes (verifies opt-in)
  • `when({ intro: true })` → SSR emits `enterFrom` on the rendered branch (single-slot case)
• Full suite: 723 pass / 2 skipped (was 720 / 2)
• Manual verification on the portfolio site — the flash should be gone

Note on user CSS

For this to give the expected visual result, users need the animation's transition to be set up in CSS. The recommended pattern (which the JSDoc examples use) is:

```ts
animate: {
enterFrom: "opacity-0 translate-y-4",
enter: "opacity-100 translate-y-0 transition duration-300",
}
```

The `transition` on `enter` is what makes the class-swap animate rather than snap.

🤖 Generated with Claude Code

[cloudflare-workers-and-pages]

Deploying effex with    Cloudflare Pages

Latest commit:	65c74ec
Status:	✅  Deploy successful!
Preview URL:	https://ce1cfc68.effex.pages.dev
Branch Preview URL:	https://feat-ssr-emit-enter-from.effex.pages.dev

View logs

[cloudflare-workers-and-pages]

Deploying effex-api with    Cloudflare Pages

Latest commit:	65c74ec
Status:	✅  Deploy successful!
Preview URL:	https://0654de3a.effex-api.pages.dev
Branch Preview URL:	https://feat-ssr-emit-enter-from.effex-api.pages.dev

View logs