fix(rules): forbid a diagonal 5-foot step past a corner

PF1e (SRD Measuring Distance): "You can't move diagonally past a corner (even by taking a 5-foot step)." _step5_to now rejects a diagonal destination whose two flanking squares are not both passable, reusing Grid.diagonal_allowed - the same corner rule the regular movement already applies. README updated.

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

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

src/pf1e_simulator/combat.py

@@ -5777,7 +5777,11 @@ class CombatEngine:
             return ActionResult(kind=kind, ok=True, reason=_REASON_OK)
         adjacent = self._grid.distance(state.pos, destination) == 1
         free = self._cells_free(self._footprint_at(destination, state), state)
-        if not adjacent or not free:
+        # PF1e (SRD Measuring Distance): "You can't move diagonally past a
+        # 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:
             self._log(
                 f"round {self._current_round} {state.combatant.id}: "
                 f"{kind} rejected ({_REASON_UNREACHABLE_DESTINATION})"