fix(actions): add downgraded:immediate_swift to ACTION_RESULT_REASONS (emitted since conformance L10 but absent from the published vocabulary); grep showed no other emitted-but-missing ActionResult reason (no_charge_path is a ChargeResult field) - drift guard test pins every _REASON_* constant against the frozenset

src/pf1e_simulator/combat.py

@@ -1111,7 +1111,7 @@ ACTION_RESULT_REASONS: frozenset[str] = frozenset({
     "no_line_of_effect", "no_slot", "insufficient_resource",
     "downgraded:staggered", "downgraded:nauseated",
     "downgraded:action_economy", "downgraded:cast_time",
-    "downgraded:surprise",
+    "downgraded:surprise", "downgraded:immediate_swift",
     "delayed", "already_delayed", "ready",
     "not_prone", "already_prone", "unreachable_destination",
     "weapon_unavailable", "invalid_destination", "cannot_combine",

tests/test_immediate_actions.py

@@ -10,7 +10,9 @@ from __future__ import annotations
 
 from typing import Literal
 
+import pf1e_simulator.combat as combat_mod
 from pf1e_simulator.combat import (
+    ACTION_RESULT_REASONS,
     Action,
     CombatantState,
     CombatEngine,
@@ -497,3 +499,16 @@ def test_damage_taken_reaction_fires() -> None:
     result = engine.run()
     assert any("reactor: short bow vs caster" in line for line in result.transcript)
     assert caster_state.hp < 50
+
+
+def test_downgraded_immediate_swift_is_in_action_result_reasons() -> None:
+    """The reason vocabulary published to policies must contain every reason the
+    engine emits: downgraded:immediate_swift (swift dropped because an
+    in-turn immediate spent it) was emitted but missing from the frozenset."""
+    assert "downgraded:immediate_swift" in ACTION_RESULT_REASONS
+    emitted = {
+        value
+        for name, value in vars(combat_mod).items()
+        if name.startswith("_REASON_") and isinstance(value, str)
+    }
+    assert emitted <= set(ACTION_RESULT_REASONS)