Make NPCs a dedicated section with per-NPC stats

Replace the single shared `npc` stat template (+ npc_card_types) with an
`npcs` section: each NPC keyed by a stable id, carrying its own name,
description, trigger keys, and its OWN stats block. The AI addresses NPCs
as npc.<id>.<stat> (id shown in context), which also fixes the old
card-id-guessing problem. On adventure creation each NPC auto-creates a
story card (name/keys/desc) for lore + in-scene detection, unless a
same-name card already exists. All NPCs instantiate up front.

- engine: npcs instantiate/apply/render/reference, npc_name/npc_triggers
- builder: _visible_npcs matches each NPC's own keys
- create_adventure: auto-create story cards from npcs
- WorldStateDrawer: render defined NPCs with their own stats + desc tooltip
- demo seed: Gwen (health/trust) + Bandit Leader (health/aggression)
- tests updated (34 pass); no new migration (npcs lives in stat_schema JSON)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/app/worldstate/__init__.py

@@ -8,7 +8,8 @@ from .engine import (
     extract_delta,
     has_schema,
     instantiate,
-    npc_types,
+    npc_name,
+    npc_triggers,
     render_reference,
     render_state_section,
 )
@@ -20,7 +21,8 @@ __all__ = [
     "extract_delta",
     "has_schema",
     "instantiate",
-    "npc_types",
+    "npc_name",
+    "npc_triggers",
     "render_reference",
     "render_state_section",
 ]