fix(map): load_map rejects two deployment sides bound to the same zone letter (build_states resets its placed set per side, so shared letters deployed camps on top of each other) - fail-fast MapValidationError + README rule, reject/accept tests

tests/test_map.py

@@ -109,3 +109,20 @@ def test_rejects_deployment_without_zone(tmp_path: Path) -> None:
     body = _minimal_map() + "zones: |\n  ...\n  .A.\n  ...\ndeployment:\n  players: B\n"
     with pytest.raises(MapValidationError):
         load_map(_write(tmp_path, body))
+
+
+def test_rejects_shared_deployment_zone_letter(tmp_path: Path) -> None:
+    """Two sides bound to the same zone letter would deploy on top of each
+    other (build_states resets its placed set per side): a data error."""
+    body = _minimal_map() + (
+        "zones: |\n  ...\n  .A.\n  ...\ndeployment:\n  players: A\n  monsters: A\n"
+    )
+    with pytest.raises(MapValidationError, match="share zone letters"):
+        load_map(_write(tmp_path, body))
+
+
+def test_accepts_distinct_deployment_zone_letters(tmp_path: Path) -> None:
+    body = _minimal_map() + (
+        "zones: |\n  ...\n  AB.\n  ...\ndeployment:\n  players: A\n  monsters: B\n"
+    )
+    assert load_map(_write(tmp_path, body)).deployment == {"players": "A", "monsters": "B"}