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>

content/lore/canon-roadmap.md

@@ -25,6 +25,14 @@ order the Margreave is being written. Source of truth is `content/lore/*.md`;
   example. Not game canon. The Duncarrow scaffolding (Crell chain, Mera Fenn) is
   deleted.
 
+## Lesson from the Duncarrow purge
+
+Generated content under `content/world/` has a client-side consumer:
+`client/scripts/content/content_db.gd` (loaded via `client/scripts/harness/npc_harness.gd`)
+and its GUT tests in `client/tests/unit/test_content_db.gd`. Deleting generated
+content — a whole namespace of topics/npcs/canon — is a **client change**, not
+just a content change; check `client/` before purging.
+
 ## Pending (not yet authored)
 
 - Per-town knowledge that grounds the cosmology: possessed NPCs, blood-harvest