conformance: block 5-foot step on difficult terrain (SRD Take 5-Foot Step)

A 5-foot step is hampered by difficult terrain: stepping out of OR into a
difficult cell is blocked. _step5 (auto-directed) logs 'wait (cannot step)';
_step5_to (explicit destination) rejects with unreachable_destination.
Engine-level tests cover destination, auto-path, and origin cases.

src/pf1e_simulator/combat.py

@@ -6898,8 +6898,17 @@ class CombatEngine:
         if self.has_condition_flag(state, "cant_move"):
             self._log(f"round {self._current_round} {state.combatant.id}: wait (cannot move)")
             return
+        # SRD "Take 5-Foot Step": "You can only take a 5-foot-step if your
+        # movement isn't hampered by difficult terrain or darkness." — a step
+        # out of OR into a difficult cell is blocked (darkness not modeled).
+        if self._footprint_touches_difficult(self._footprint_at(state.pos, state)):
+            self._log(f"round {self._current_round} {state.combatant.id}: wait (cannot step)")
+            return
         path = self._move_path(state, target)
         start = state.pos
+        if self._footprint_touches_difficult(self._footprint_at(path[0], state)):
+            self._log(f"round {self._current_round} {state.combatant.id}: wait (cannot step)")
+            return
         state.pos = path[0]
         state.moved_this_turn = True
         self._log(
@@ -6921,7 +6930,14 @@ class CombatEngine:
         # corner (even by taking a 5-foot step)."
         diagonal = state.pos[0] != destination[0] and state.pos[1] != destination[1]
         corner_clear = not diagonal or self._grid.diagonal_allowed(state.pos, destination)
-        if not adjacent or not free or not corner_clear:
+        # SRD "Take 5-Foot Step": "You can only take a 5-foot-step if your
+        # movement isn't hampered by difficult terrain or darkness." — a step
+        # out of OR into a difficult cell is blocked (darkness not modeled).
+        step_clear = not (
+            self._footprint_touches_difficult(self._footprint_at(state.pos, state))
+            or self._footprint_touches_difficult(self._footprint_at(destination, state))
+        )
+        if not adjacent or not free or not corner_clear or not step_clear:
             self._log(
                 f"round {self._current_round} {state.combatant.id}: "
                 f"{kind} rejected ({_REASON_UNREACHABLE_DESTINATION})"