fix(rules): stop auto-directed movement cutting corners

PF1e (CRB, Measuring Distance): "You can't move diagonally past a corner
(even by taking a 5-foot step)". _best_move_step (the greedy descent behind
the auto-directed move/run/5-foot-step) now enforces the same
diagonal_allowed corner check _step5_to and _best_flee_step already apply.

tests/test_combat.py::test_ranged_no_line_of_effect_moves_around_wall
pinned the old corner-cutting step (0,1)->(1,2) past the wall at (0,2);
re-scripted for the legal (0,1)->(1,1) path (wall cover now applies).

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

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

tests/test_move_destination.py

@@ -378,6 +378,37 @@ def test_step5_diagonal_with_clear_flanks_is_allowed() -> None:
     assert eng._states[0].pos == (1, 1)
 
 
+def test_auto_move_does_not_cut_wall_corner() -> None:
+    # SRD Measuring Distance: "You can't move diagonally past a corner."
+    # Wall at (1, 0) flanks the (0,0)->(1,1) diagonal: the auto-directed
+    # move must walk around it (via (0, 1)), never through the corner.
+    def policy(_engine: CombatEngine, state: CombatantState) -> tuple[Action, ...]:
+        if state.combatant.id == "hero":
+            return (Action(kind="move", target_id="foe"),)
+        return _WAIT
+
+    eng = _hero_vs_foe(policy, [10, 5], round_cap=1, foe_pos=(2, 2), grid=_corner_grid())
+    t = eng.run().transcript
+    move_line = next(ln for ln in t if "hero: move" in ln)
+    assert "(0,0)->(1,1)" not in move_line  # no corner cut
+    assert "(0,0)->(0,1)" in move_line  # went around the wall
+
+
+def test_auto_step5_does_not_cut_wall_corner() -> None:
+    # The auto 5-foot step takes its single step from the same greedy walk,
+    # so it too must refuse the corner-cutting diagonal toward the foe.
+    def policy(_engine: CombatEngine, state: CombatantState) -> tuple[Action, ...]:
+        if state.combatant.id == "hero":
+            return (Action(kind="5foot_step", target_id="foe"),)
+        return _WAIT
+
+    eng = _hero_vs_foe(policy, [10, 5], round_cap=1, foe_pos=(2, 2), grid=_corner_grid())
+    t = eng.run().transcript
+    assert "round 1 hero: 5ft step (0,0)->(1,1)" not in t
+    assert "round 1 hero: 5ft step (0,0)->(0,1)" in t
+    assert eng._states[0].pos == (0, 1)
+
+
 # ── (e) U1 preserved: neither target nor destination → missing_target ─────────