Phase 1 of #7 only embedded the data island when the stored content
type was application/ld+json; .ttl/.n3 URLs got the mashlib wrapper
without an embedded payload.

Extend the embed branch to cover text/turtle and text/n3:

- Try JSON.parse first. JSS converts Turtle/N3 to JSON-LD on PUT
  (handlePut conneg branch), so for HTTP-PUT'd .ttl/.n3 files the
  bytes on disk are already JSON-LD — pass the Buffer through, no
  extra string copy.
- Fall back to turtleToJsonLd (n3.js) for files placed on the
  filesystem out-of-band in their native format. JSON.stringify the
  result compactly so it doesn't burn cap bytes on whitespace.
- Both parses failing → drop the island silently. The wrapper still
  renders and mashlib XHR-fetches the original as before. Other RDF
  formats (rdf+xml, etc.) are untouched.

Add integration coverage: PUT Turtle / PUT N3 → GET text/html shows
the embedded JSON-LD with the original literals. The
both-parses-fail branch can't be reached via HTTP (PUT validates and
rejects malformed Turtle with 400 before storage), so it's covered
by inspection only.

All 555 tests pass (553 + 2 new).

Three real points addressed:

1. Use `RDF_TYPES.JSON_LD` / `.TURTLE` / `.N3` instead of hardcoded
   strings. Already imported and used elsewhere in this file.

2. Avoid double-decode in the JSON-LD-on-disk hot path. Restructure
   the embed branch so:
     - `application/ld+json` URL: pass Buffer through (no decode,
       no parse — matches pre-PR behaviour).
     - `text/turtle` / `text/n3` URL: decode once, JSON.parse;
       on success pass the decoded *text* through (not the Buffer)
       so dataIsland() doesn't re-decode via String() coercion.
     - Turtle parse fallback unchanged.

3. Add coverage for the both-parses-fail branch. The handler's
   defensive guard fires when a `.ttl`/`.n3` file is placed on disk
   out-of-band (bypassing the PUT validator). The new test plants
   a malformed Turtle file directly into the test data dir via
   node:fs and asserts the wrapper still renders without an island.

556 / 556 tests pass (553 + 3 new).