feat(narrative): add StoryStore for file-based narrative persistence

Three JSON files per simulation under narrative/ subdir:
- story_beats.json: chronological story passages
- translator_state.json: per-platform file offsets
- characters.json: extended character profiles

Tests: 4/4 passing.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/services/narrative/story_store.py

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+"""File-based persistence for narrative state.
+
+Each simulation gets a `narrative/` subdirectory inside its data dir,
+containing three files:
+  - story_beats.json: chronological list of generated story passages
+  - translator_state.json: tracks file offset per platform's actions.jsonl
+  - characters.json: extended character profiles with emotional state
+"""
+import os
+import json
+from typing import Optional
+
+
+class StoryStore:
+    """Manages narrative/*.json files for a single simulation."""
+
+    def __init__(self, sim_dir: str):
+        self.sim_dir = sim_dir
+        self.narrative_dir = os.path.join(sim_dir, "narrative")
+        self.beats_path = os.path.join(self.narrative_dir, "story_beats.json")
+        self.translator_state_path = os.path.join(
+            self.narrative_dir, "translator_state.json"
+        )
+        self.characters_path = os.path.join(self.narrative_dir, "characters.json")
+
+    def _ensure_dir(self) -> None:
+        os.makedirs(self.narrative_dir, exist_ok=True)
+
+    def append_beat(self, beat: dict) -> None:
+        self._ensure_dir()
+        beats = self.get_all_beats()
+        beats.append(beat)
+        with open(self.beats_path, "w", encoding="utf-8") as f:
+            json.dump(beats, f, ensure_ascii=False, indent=2)
+
+    def get_all_beats(self) -> list:
+        if not os.path.exists(self.beats_path):
+            return []
+        with open(self.beats_path, "r", encoding="utf-8") as f:
+            return json.load(f)
+
+    def get_beat_by_round(self, round_num: int) -> Optional[dict]:
+        for beat in self.get_all_beats():
+            if beat.get("round") == round_num:
+                return beat
+        return None
+
+    def get_file_offset(self, platform: str) -> int:
+        if not os.path.exists(self.translator_state_path):
+            return 0
+        with open(self.translator_state_path, "r", encoding="utf-8") as f:
+            state = json.load(f)
+        return state.get(f"{platform}_offset", 0)
+
+    def set_file_offset(self, platform: str, offset: int) -> None:
+        self._ensure_dir()
+        state = {}
+        if os.path.exists(self.translator_state_path):
+            with open(self.translator_state_path, "r", encoding="utf-8") as f:
+                state = json.load(f)
+        state[f"{platform}_offset"] = offset
+        with open(self.translator_state_path, "w", encoding="utf-8") as f:
+            json.dump(state, f, indent=2)
+
+    def save_characters(self, characters: list) -> None:
+        self._ensure_dir()
+        with open(self.characters_path, "w", encoding="utf-8") as f:
+            json.dump(characters, f, ensure_ascii=False, indent=2)
+
+    def load_characters(self) -> list:
+        if not os.path.exists(self.characters_path):
+            return []
+        with open(self.characters_path, "r", encoding="utf-8") as f:
+            return json.load(f)

backend/tests/test_story_store.py

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+import os
+import tempfile
+import pytest
+from app.services.narrative.story_store import StoryStore
+
+
+@pytest.fixture
+def temp_sim_dir():
+    with tempfile.TemporaryDirectory() as d:
+        sim_dir = os.path.join(d, "sim_test123")
+        os.makedirs(sim_dir)
+        yield sim_dir
+
+
+def test_save_and_load_story_beats(temp_sim_dir):
+    store = StoryStore(temp_sim_dir)
+    beat = {"round": 1, "prose": "Elena spoke.", "characters": ["elena"]}
+    store.append_beat(beat)
+
+    beats = store.get_all_beats()
+    assert len(beats) == 1
+    assert beats[0]["prose"] == "Elena spoke."
+
+
+def test_translator_state_tracks_offset(temp_sim_dir):
+    store = StoryStore(temp_sim_dir)
+    assert store.get_file_offset("twitter") == 0
+
+    store.set_file_offset("twitter", 1024)
+    assert store.get_file_offset("twitter") == 1024
+
+
+def test_get_beat_by_round(temp_sim_dir):
+    store = StoryStore(temp_sim_dir)
+    store.append_beat({"round": 1, "prose": "First"})
+    store.append_beat({"round": 2, "prose": "Second"})
+
+    beat = store.get_beat_by_round(2)
+    assert beat["prose"] == "Second"
+
+
+def test_narrative_dir_created_on_first_write(temp_sim_dir):
+    store = StoryStore(temp_sim_dir)
+    store.append_beat({"round": 1, "prose": "test"})
+
+    narrative_dir = os.path.join(temp_sim_dir, "narrative")
+    assert os.path.isdir(narrative_dir)