Use the server's renamed registry capability, urn:inbuxa:jmap:registry

inbuxa-server renames the identifiers that carried the upstream name (its
SPEC.md §2.4). Upstream's capability for the registry (x:) objects is now
urn:inbuxa:jmap:registry, beside the fork's own urn:inbuxa:jmap, which is
unchanged. There's no alias, so this lands with the server change and
deploys with it. The mock advertises the new name too. No user-visible
strings change.

server/src/mock/config.ts

@@ -5,7 +5,7 @@ export const PERMISSION_SNAPSHOT = (JSON.parse(readFileSync(new URL("../../../we
 
 export const PORT = Number(process.env.MOCK_PORT ?? 8788);
 /**
- * Omit `urn:stalwart:jmap` from the session, so a sign-in can be tested
+ * Omit `urn:inbuxa:jmap:registry` from the session, so a sign-in can be tested
  * against a server ihasmail does not support. This is only that: the rest of
  * the mock still behaves like 0.16. Emulating 0.15 properly went with the
  * support for it.