Add Phase 12: native RPG world state

Structured world/player/NPC stats, two-way flags, and sticky milestones
per scenario (stat_schema). The AI proposes a per-turn delta; a Python
engine referees it (clamp to min/max, per-turn cap, cooldown, counters).
Band word-labels plus a fixed stat guide (descriptions + full ranges)
keep the model grounded. World State drawer + Insights delta report;
undo/retry roll it back via the Phase 11 snapshot pattern.

- migrations 26-28 (scenarios.stat_schema, adventures.world_state,
  actions.world_state_before); all nullable, additive, safe on existing rows
- migration 29 raises the default context budget 4096 -> 16384
  (custom values preserved)
- seeded demo scenario 04-rpg-world-state.json (Bandit Camp)
- 19 new tests (33 total pass)

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

backend/app/worldstate/__init__.py

@@ -0,0 +1,26 @@
+"""Phase 12 — RPG world state: the AI proposes stat/milestone deltas, this
+module validates and clamps them against a scenario's stat_schema."""
+
+from .engine import (
+    EMIT_RULE,
+    apply_delta,
+    band_label,
+    extract_delta,
+    has_schema,
+    instantiate,
+    npc_types,
+    render_reference,
+    render_state_section,
+)
+
+__all__ = [
+    "EMIT_RULE",
+    "apply_delta",
+    "band_label",
+    "extract_delta",
+    "has_schema",
+    "instantiate",
+    "npc_types",
+    "render_reference",
+    "render_state_section",
+]