Keep world-state emit reliable across turns

The AI would sometimes stop emitting the `state` delta block once it missed
a turn. Two compounding causes: the emit rule sat only in the system block
(far from where the model generates), and the block was stripped before
storage — so every replayed history turn looked blockless, biasing the model
by imitation to stop emitting too.

- EMIT_REMINDER: a one-line reminder appended last in the prompt (strongest
  recency slot), gated on has_ws and counted against the token budget.
- render_delta_block + _history_text: re-attach each past AI turn's own delta
  block in replayed history (reconstructed from the stored snapshot delta), so
  the model always sees its emit format. Action.text stays clean, so UI,
  embeddings, and card/NPC trigger-matching are unaffected. History budgeting
  counts the augmented text so it can't overflow.

Undo/retry untouched (read the separate world_state_before column). 46 tests.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UWVyFKvqJGjfbXdibLgkMe

backend/app/worldstate/__init__.py

@@ -2,6 +2,7 @@
 module validates and clamps them against a scenario's stat_schema."""
 
 from .engine import (
+    EMIT_REMINDER,
     EMIT_RULE,
     apply_delta,
     apply_override,
@@ -11,11 +12,13 @@ from .engine import (
     instantiate,
     npc_name,
     npc_triggers,
+    render_delta_block,
     render_reference,
     render_state_section,
 )
 
 __all__ = [
+    "EMIT_REMINDER",
     "EMIT_RULE",
     "apply_delta",
     "apply_override",
@@ -25,6 +28,7 @@ __all__ = [
     "instantiate",
     "npc_name",
     "npc_triggers",
+    "render_delta_block",
     "render_reference",
     "render_state_section",
 ]