fix(content): repair client consumers broken by the Duncarrow purge

The purge plan assumed content/world/** was pure build output. The client
reads it: content_db.gd loads content/world/topics/ (now absent, since every
topic was Duncarrow-generated) and its tests asserted on deleted ids.

- content_db: a missing content dir is an empty dir, not an error (§13)
- test_content_db: repoint at cosmology canon; drop deleted npc.mera-fenn
- test_content_db: assert the no-body-on-client invariant structurally, so it
  cannot pass vacuously when the topic set is empty
- schema.md: finish repointing the worked example onto specimen.md's ids
- restore a local real-bible --check smoke test (was only in CI)
- roadmap: record that generated content has client consumers

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

client/scripts/content/content_db.gd

@@ -39,7 +39,8 @@ func _load_dir(dir_path: String) -> Dictionary:
 	var out: Dictionary = {}
 	var dir := DirAccess.open(dir_path)
 	if dir == null:
-		push_error("content dir not found: %s" % dir_path)
+		# A missing content dir means "no content of this kind authored yet",
+		# not an error — e.g. content/world/topics/ before any gated content exists.
 		return out
 	for file in dir.get_files():
 		if not file.ends_with(".json"):