feat(collection): add slugFields for multi-segment collection paths

[tihu220]

Resolves #340

This implements the multi-slug API proposed by @JedWatson in the issue — keeping collection paths static and deterministic rather than resolving paths via a callback, so the on-disk location of entries is always known from config.

API

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

• Each entry of slugFields maps onto a * segment of path, in order.
• The slug of an entry is the segment values joined with a slash (en/my-post), so tree listing, routing, the reader and both storage backends work unchanged.
• Fully backwards compatible: collections using slugField alone behave exactly as before.

Behaviour notes

• Config validation: the number of * segments must equal slugFields.length, every entry must be a slug-form field, and the last entry must be the slugField (the segment shown as "the slug" in the admin UI).
• Secondary slug fields validate as slug segments (no slashes, no ..); the primary field is additionally checked for uniqueness against the composite slug, so en/my-post and fr/my-post can co-exist while duplicate paths are still rejected.
• Duplicating an entry appends -copy to the primary segment.
• The reader accepts composite slugs: reader.collections.blog.read('en/my-post'), and list() returns composite slugs.
• Existing ** glob collections are unaffected; combining slugFields with ** is a config error.

Testing

• New unit tests in packages/keystatic/src/app/slug-fields.test.ts cover config validation, composite path mapping, slug computation, tree listing at exact depth (including ignoring too-deep entries) and reader list/read for both multi- and single-slug collections.
• Typecheck is clean; the full unit suite shows no regressions.

/claim #340

[changeset-bot]

🦋 Changeset detected

Latest commit: 588447c

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 4 packages

Name	Type
@keystatic/core	Minor
@keystatic/templates-astro	Patch
@keystatic/templates-nextjs	Patch
@keystatic/templates-remix	Patch

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[tihu220]

/claim #340