Current behaviour

createPodStructure in `src/handlers/container.js` writes three RDF files when a pod is created:

```
settings/Preferences.ttl
settings/publicTypeIndex.ttl
settings/privateTypeIndex.ttl
```

The `.ttl` extension is chosen explicitly "for mashlib" (per the inline comment), but JSS is otherwise a JSON-LD-native server — Turtle support is opt-in via `--conneg`, while `.jsonld` / `application/ld+json` is the always-on default.

Proposal

Change the three artefacts to:

```
settings/prefs.jsonld
settings/publicTypeIndex.jsonld
settings/privateTypeIndex.jsonld
```

• The `Preferences.ttl` → `prefs.jsonld` rename also aligns with the `pim:preferencesFile` predicate on the WebID; both mashlib and solid-client historically point at a variety of paths.
• Serialize with the existing JSON-LD writer — no new dependency.
• Keep `profile/card` as HTML (already the standard Solid profile-as-HTML pattern, unchanged).

Why it matters

Client apps that do "follow your nose" discovery and want to write to these files (add a TypeIndex registration, update a preference) have to pick one extension. Any mismatch between a pod's freshly-created `.ttl` and the client's assumed `.jsonld` produces two files with the same logical content and a broken chain — I hit exactly this when wiring up pilot's "Create prefs" / "Create TypeIndex" buttons against a JSS-created pod. Ended up renaming server-side files and updating WebID predicates to unblock.

`.jsonld` is also a lot easier to hand-edit (stable key order, no namespace dance), which matters when a user is curating a growing TypeIndex — `.ttl`'s shorthand looks great for reading but loses parity with how pilot / solid-panes / most new clients write.

Suggested changes

`createPodStructure` in `src/handlers/container.js`:

```diff

• await storage.write(`${podPath}settings/Preferences.ttl`, serialize(prefs));

• await storage.write(`${podPath}settings/prefs.jsonld`, JSON.stringify(prefs, null, 2));

• const publicTypeIndex = generateTypeIndex(`${podUri}settings/publicTypeIndex.ttl`);
• await storage.write(`${podPath}settings/publicTypeIndex.ttl`, serialize(publicTypeIndex));

• const publicTypeIndex = generateTypeIndex(`${podUri}settings/publicTypeIndex.jsonld`);
• await storage.write(`${podPath}settings/publicTypeIndex.jsonld`, JSON.stringify(publicTypeIndex, null, 2));

• const privateTypeIndex = generateTypeIndex(`${podUri}settings/privateTypeIndex.ttl`);
• await storage.write(`${podPath}settings/privateTypeIndex.ttl`, serialize(privateTypeIndex));

• const privateTypeIndex = generateTypeIndex(`${podUri}settings/privateTypeIndex.jsonld`);
• await storage.write(`${podPath}settings/privateTypeIndex.jsonld`, JSON.stringify(privateTypeIndex, null, 2));
  ```

Same for `createRootPodStructure` in `src/server.js` (single-user root-pod mode).

The WebID card keeps `pim:preferencesFile` / `solid:publicTypeIndex` pointing at the new paths so follow-your-nose continues to work.

Back-compat

Breaks pods that external clients assume are `.ttl`-shaped. Three options, in order of my preference:

1. Just change it. JSS is `0.0.x`, moving fast, and the dominant consumers (mashlib + conneg clients) can already fetch `.jsonld` fine.
2. Write both (`prefs.jsonld` AND `Preferences.ttl`) for a release or two, then drop `.ttl`. Ugly but safe.
3. Add a flag like `--pod-format jsonld|ttl` defaulting to `jsonld` for new pods.

Happy to PR whichever direction you pick.