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>

tests/test_move_destination.py

@@ -246,6 +246,44 @@ def test_step5_to_occupied_destination_is_rejected() -> None:
     assert eng._states[0].pos == (0, 0)
 
 
+def _corner_grid() -> Grid:
+    """Open 8x8 floor with a wall at (1, 0) — a corner for the (0,0)->(1,1) step."""
+    rows = ["." * 8, "#" + "." * 7] + ["." * 8] * 6
+    legend = {
+        ".": TerrainType(type="floor", move_cost=1),
+        "#": TerrainType(type="wall", move_cost=None, blocks_los=True),
+    }
+    spec = MapSpec(name="corner", terrain=tuple(rows), legend=legend)
+    return Grid.from_spec(spec)
+
+
+def test_step5_diagonal_past_corner_is_rejected() -> None:
+    # SRD Measuring Distance: "You can't move diagonally past a corner
+    # (even by taking a 5-foot step)." Wall at (1, 0) blocks the flank.
+    def policy(_engine: CombatEngine, state: CombatantState) -> tuple[Action, ...]:
+        if state.combatant.id == "hero":
+            return (Action(kind="5foot_step", destination=(1, 1)),)
+        return _WAIT
+
+    eng = _hero_vs_foe(policy, [10, 5], round_cap=1, foe_pos=(7, 7), grid=_corner_grid())
+    t = eng.run().transcript
+    assert "round 1 hero: 5foot_step rejected (unreachable_destination)" in t
+    assert eng._states[0].pos == (0, 0)
+
+
+def test_step5_diagonal_with_clear_flanks_is_allowed() -> None:
+    # The same diagonal step on an open grid (both flanks passable) is legal.
+    def policy(_engine: CombatEngine, state: CombatantState) -> tuple[Action, ...]:
+        if state.combatant.id == "hero":
+            return (Action(kind="5foot_step", destination=(1, 1)),)
+        return _WAIT
+
+    eng = _hero_vs_foe(policy, [10, 5], round_cap=1, foe_pos=(7, 7))
+    t = eng.run().transcript
+    assert "round 1 hero: 5ft step (0,0)->(1,1)" in t
+    assert eng._states[0].pos == (1, 1)
+
+
 # ── (e) U1 preserved: neither target nor destination → missing_target ─────────