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.

tests/test_move_destination.py

@@ -450,3 +450,74 @@ def test_move_with_target_auto_paths_as_before() -> None:
     assert eng._states[0].last_action_results == [
         ActionResult(kind="move", ok=True, reason="ok")
     ]
+
+
+# ── (g) 5-foot step blocked by difficult terrain ────────────────────────────
+
+
+def _rubble_adjacent_grid() -> Grid:
+    """8x8 grid where (0, 1) is difficult terrain (rubble, cost 2)."""
+    rows = [".~" + "." * 6] + ["." * 8] * 7
+    legend = {
+        ".": TerrainType(type="floor", move_cost=1),
+        "~": TerrainType(type="rubble", move_cost=2),
+    }
+    return Grid.from_spec(MapSpec(name="rubble", terrain=tuple(rows), legend=legend))
+
+
+def test_step5_to_difficult_destination_is_rejected() -> None:
+    """SRD Take 5-Foot Step: hampered by difficult terrain blocks the step."""
+
+    def policy(_engine: CombatEngine, state: CombatantState) -> tuple[Action, ...]:
+        if state.combatant.id == "hero":
+            return (Action(kind="5foot_step", destination=(0, 1)),)
+        return _WAIT
+
+    grid = _rubble_adjacent_grid()
+    eng = _hero_vs_foe(policy, [10, 5], round_cap=1, foe_pos=(0, 4), grid=grid)
+    t = eng.run().transcript
+    assert eng._states[0].pos == (0, 0)
+    assert "round 1 hero: 5foot_step rejected (unreachable_destination)" in t
+
+
+def test_step5_auto_path_to_difficult_is_blocked() -> None:
+    """SRD Take 5-Foot Step: auto-directed step into difficult terrain blocked."""
+
+    def policy(_engine: CombatEngine, state: CombatantState) -> tuple[Action, ...]:
+        if state.combatant.id == "hero":
+            return (Action(kind="5foot_step", target_id="foe"),)
+        return _WAIT
+
+    legend = {
+        ".": TerrainType(type="floor", move_cost=1),
+        "~": TerrainType(type="rubble", move_cost=2),
+        "#": TerrainType(type="wall", move_cost=None, blocks_los=True),
+    }
+    # Hero at (0,0), wall at (0,1) blocks east, rubble at (1,0) is the only exit.
+    # Foe at (2,0) -- the greedy walk must go through (1,0) rubble.
+    rows = [".#" + "." * 6, "~" + "." * 7] + ["." * 8] * 6
+    grid = Grid.from_spec(MapSpec(name="funnel", terrain=tuple(rows), legend=legend))
+    eng = _hero_vs_foe(policy, [10, 5], round_cap=1, foe_pos=(2, 0), grid=grid)
+    t = eng.run().transcript
+    assert eng._states[0].pos == (0, 0)
+    assert "round 1 hero: wait (cannot step)" in t
+
+
+def test_step5_from_difficult_origin_is_rejected() -> None:
+    """SRD Take 5-Foot Step: origin on difficult terrain also blocks the step."""
+
+    def policy(_engine: CombatEngine, state: CombatantState) -> tuple[Action, ...]:
+        if state.combatant.id == "hero":
+            return (Action(kind="5foot_step", destination=(1, 1)),)
+        return _WAIT
+
+    legend = {
+        ".": TerrainType(type="floor", move_cost=1),
+        "~": TerrainType(type="rubble", move_cost=2),
+    }
+    rows = ["~" + "." * 7] + ["." * 8] * 7
+    grid = Grid.from_spec(MapSpec(name="rubble", terrain=tuple(rows), legend=legend))
+    eng = _hero_vs_foe(policy, [10, 5], round_cap=1, foe_pos=(0, 4), grid=grid)
+    t = eng.run().transcript
+    assert eng._states[0].pos == (0, 0)
+    assert "round 1 hero: 5foot_step rejected (unreachable_destination)" in t