Add Open Props emitter (`<host>-open-props.css`)

[Manavarya09]

Add a new emitter that writes Open Props (https://open-props.style) CSS custom properties from the extracted DTCG tokens. Mirrors the existing emitters in src/formatters/.

Why

Open Props is a popular zero-runtime CSS variables library and a natural fit for designlang's output. Adding it means users can plug an extraction straight into an Open Props project without a manual translation step.

What to build

src/formatters/open-props.js exporting formatOpenProps(design) that returns a string of @layer blocks following the Open Props conventions:

@layer base {
  :root {
    --brand-1: <primary>;
    --brand-2: <secondary>;
    --size-1: <spacing[0]>;
    --size-2: <spacing[1]>;
    --font-sans: <typography.families[0]>;
    --radius-1: <radii[0]>;
    /* ...etc */
  }
}

Wire it up

1. Add formatOpenProps to the extract command's emit loop in bin/design-extract.js (search for where formatTailwind is written).
2. Add a corresponding entry to website/lib/build-files.js so the web extractor includes it too.
3. Add a unit test in tests/formatters.test.js (use the existing mockDesign fixture; assert the output contains --brand-1 and a known value).

Acceptance

• Running npx designlang stripe.com writes stripe-com-open-props.css alongside the other outputs.
• Running node --test tests/formatters.test.js passes including the new test.
• README is unchanged (we add to features list later in batch).

Scope

~60 lines of code plus a test. The closest reference implementation is src/formatters/css-vars.js.