fix(rules): block immediate actions while flat-footed

SRD 'Immediate Actions': 'You also cannot use an immediate action if you are flat-footed.' _check_reactions now skips states carrying the flat_footed condition before consulting the reaction policy. The engine never applies flat_footed itself (latent gate), so the unit test injects the condition via active_conditions, with a control test proving the same setup fires when not flat-footed.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/pf1e_simulator/combat.py

@@ -3914,6 +3914,8 @@ class CombatEngine:
         at most one trigger resolves per event (a fired readied action
         suppresses the policy loop). One immediate per event: the first active
         combatant (in state order) whose policy returns an action gets to act.
+        PF1e (SRD "Immediate Actions"): "You also cannot use an immediate
+        action if you are flat-footed" — flat-footed states are skipped.
         Reactions never cascade — while a reaction is executing, further
         events are ignored.
         """
@@ -3924,7 +3926,12 @@ class CombatEngine:
         if self._reaction_policy is None:
             return
         for state in self._states:
-            if not state.active or state.immediate_used or state.swift_used_this_turn:
+            if (
+                not state.active
+                or state.immediate_used
+                or state.swift_used_this_turn
+                or self._has_condition_named(state, "flat_footed")
+            ):
                 continue
             action = self._reaction_policy(self, state, event)
             if action is None: