Translate non-JSON-LD RDF formats to JSON-LD for HTML data island embedding #344

Summary

When serving the HTML view of a stored resource, JSS embeds a JSON-LD data island
in the response. This works for resources stored as JSON-LD. For resources stored
in Turtle or N3, the data island is currently empty.

This issue tracks adding server-side format translation so all stored RDF formats
populate the data island uniformly.

Current Behavior

Proposed Behavior

In the HTML path of src/handlers/resource.js:

1. If stored content is JSON-LD → embed as-is (current behavior).
2. If stored content is Turtle / N3 → parse (n3.js) and serialize to JSON-LD,
   then embed.
3. If parse/serialize fails → omit the data island; existing fetch-based render
   continues to work.

Scope

• src/handlers/resource.js: select translation path by content type.
• Small helper, e.g. src/rdf/toJsonLd.js.
• Tests:
  • JSON-LD passthrough unchanged
  • Turtle → JSON-LD embedded correctly
  • N3 → JSON-LD embedded correctly
  • Parse-failure falls back cleanly (no broken HTML)

Out of Scope

• Client-side consumption of the data island.
• application/rdf+xml (rare; fall back to network fetch).
• Streaming for very large resources (already capped at 256 KB).

Acceptance

• HTML view of a Turtle-stored resource emits a non-empty
  <script type="application/ld+json" id="dataisland">.
• Same for N3.
• JSON-LD passthrough behavior unchanged.
• Existing tests remain green; new tests cover the three cases above.