feat(narrative): add action-to-verb mapping for OASIS actions

New narrative package with action_mapper module that maps the 13 OASIS
action types (CREATE_POST, LIKE_POST, REPOST, etc.) to narrative verbs
and interpretations used when generating story prose.

Tests: 4/4 passing.

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

backend/app/services/narrative/__init__.py

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+"""Narrative Layer — translates OASIS simulation output into story prose."""

backend/app/services/narrative/action_mapper.py

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+"""Maps OASIS action types to narrative verbs and interpretations.
+
+Used by the Narrative Translator to convert raw simulation actions
+(`CREATE_POST`, `LIKE_POST`, etc.) into human-readable story language.
+"""
+
+ACTION_TO_VERB = {
+    "CREATE_POST": "speaks",
+    "LIKE_POST": "agrees with",
+    "REPOST": "spreads word of",
+    "QUOTE_POST": "responds to",
+    "FOLLOW": "shows loyalty to",
+    "DO_NOTHING": "observes in silence",
+    "CREATE_COMMENT": "engages with",
+    "DISLIKE_POST": "disapproves of",
+    "LIKE_COMMENT": "validates",
+    "DISLIKE_COMMENT": "dismisses",
+    "SEARCH_POSTS": "investigates",
+    "SEARCH_USER": "seeks out",
+    "MUTE": "ignores",
+}
+
+ACTION_TO_NARRATIVE = {
+    "CREATE_POST": "Character speaks, declares, or announces",
+    "LIKE_POST": "Character agrees, supports, or nods",
+    "REPOST": "Character spreads rumor, amplifies, or gossips",
+    "QUOTE_POST": "Character responds, debates, or challenges",
+    "FOLLOW": "Character allies with or shows loyalty to",
+    "DO_NOTHING": "Character reflects, observes, or waits",
+    "CREATE_COMMENT": "Character engages in dialogue",
+    "DISLIKE_POST": "Character opposes, confronts, or disapproves",
+    "LIKE_COMMENT": "Character validates a response",
+    "DISLIKE_COMMENT": "Character dismisses or mocks",
+    "SEARCH_POSTS": "Character investigates or seeks information",
+    "SEARCH_USER": "Character seeks out a specific person",
+    "MUTE": "Character avoids, ignores, or shuns",
+}
+
+
+def map_action_to_verb(action_type: str) -> str:
+    """Return a narrative verb phrase for an OASIS action type."""
+    return ACTION_TO_VERB.get(action_type, "does something")
+
+
+def get_narrative_context(action_type: str) -> str:
+    """Return a longer narrative interpretation for an OASIS action type."""
+    return ACTION_TO_NARRATIVE.get(action_type, "Character takes an unknown action")

backend/tests/test_action_mapper.py

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+from app.services.narrative.action_mapper import map_action_to_verb, get_narrative_context
+
+
+def test_create_post_maps_to_speech():
+    result = map_action_to_verb("CREATE_POST")
+    assert result == "speaks"
+
+
+def test_like_post_maps_to_agreement():
+    result = map_action_to_verb("LIKE_POST")
+    assert result == "agrees with"
+
+
+def test_unknown_action_returns_fallback():
+    result = map_action_to_verb("UNKNOWN_ACTION")
+    assert result == "does something"
+
+
+def test_get_narrative_context_returns_interpretation():
+    ctx = get_narrative_context("REPOST")
+    assert "rumor" in ctx.lower() or "amplifies" in ctx.lower()