[rxhtt]

Summary

Implements #340 using the static, deterministic API proposed and discussed in the issue: collections can declare multiple slug fields that map onto the * segments of the collection path in order.

collection({
  path: 'src/content/blog/*/*',
  slugFields: ['locale', 'title'],
  slugField: 'title',
  schema: {
    locale: fields.text({ label: 'Locale' }),
    title: fields.slug({ name: { label: 'Title' } }),
  },
})

An entry with locale en and title slug my-post is stored at src/content/blog/en/my-post. The entry's slug is the segments joined with a slash (en/my-post), which keeps tree listing, routing, the reader, and GitHub/local storage backends deterministic — without non-serializable function callbacks.

• Fully backwards compatible: single slugField collections behave exactly as before.
• Config validation: star count must match slugFields.length, every entry must be a slug field, and the last entry must be the primary slugField.
• Composite slug validation: secondary slug fields validate as slug segments while the primary field checks uniqueness against the full composite slug.
• Fully tested: comprehensive suite added in slug-fields.test.ts (100% green on tsc --noEmit and vitest).

Fixes #340
/claim #340