refactor(storage): gate clone() behind an opt-in allowlist; expand duplicate tests

Follow-up polish from the duplicate-subgraph review:

- BaseSqliteStore.clone() now requires an opt-in `_cloneable` flag (default
  off), so a new or secret-bearing store can never be cloned by accident —
  defence-in-depth on top of the endpoint's `_DUPLICABLE_KINDS` restriction.
  Only value-source and colour-strip stores are flagged.
- Fix `_check_name_unique` annotation (`str | None = None`).
- Add tests: `layers[].brightness_source_id` remap, the warnings safety net
  firing, the clone allowlist invariant, and clone() refusing a non-cloneable
  store.

server/tests/api/test_graph_duplicate.py

@@ -94,6 +94,35 @@ def test_duplicate_remaps_bindable_slot_to_cloned_value_source(stores):
     assert c_new["intensity"]["source_id"] == res["id_map"][vs.id]
 
 
+def test_duplicate_remaps_layer_brightness_source(stores):
+    """A composite layer's value-source brightness binding (list + value ref) is
+    remapped when that value source is also in the selection."""
+    css, vss = stores
+    vs = vss.create_source(name="Dim", source_type="static", value=0.3)
+    leaf = css.create_source(name="Leaf", source_type="single_color", colors=[[9, 9, 9]])
+    comp = css.create_source(
+        name="Comp",
+        source_type="composite",
+        layers=[{**_layer(leaf.id), "brightness_source_id": vs.id}],
+    )
+    res = _duplicate_subgraph([comp.id, leaf.id, vs.id], " (copy)")
+    layer = serialize_entity(css.get(res["id_map"][comp.id]))["layers"][0]
+    assert layer["source_id"] == res["id_map"][leaf.id]
+    assert layer["brightness_source_id"] == res["id_map"][vs.id]
+
+
+def test_duplicate_safety_net_flags_unremapped_ref(stores, monkeypatch):
+    """If a reference somehow isn't remapped, the post-clone safety net reports
+    it in `warnings` rather than silently leaving two pipelines sharing a node."""
+    css, _vss = stores
+    b = css.create_source(name="B", source_type="single_color", colors=[[1, 2, 3]])
+    a = css.create_source(name="A", source_type="composite", layers=[_layer(b.id)])
+    # Force remap to a no-op so the clone keeps pointing at the original in-set id.
+    monkeypatch.setattr("ledgrab.api.routes.graph.remap_refs", lambda *a, **k: 0)
+    res = _duplicate_subgraph([a.id, b.id], " (copy)")
+    assert any(w["id"] == res["id_map"][a.id] for w in res["warnings"])
+
+
 def test_duplicate_value_source(stores):
     _css, vss = stores
     v = vss.create_source(name="V", source_type="static", value=0.5)
@@ -120,3 +149,23 @@ def test_duplicate_name_collision_is_suffixed(stores):
     res = _duplicate_subgraph([v.id], " (copy)")
     new = vss.get(res["id_map"][v.id])
     assert new.name == "V (copy) 2"
+
+
+def test_clone_allowlist_invariant():
+    """Only explicitly-flagged (secret-free) stores are cloneable; the base
+    default is off so a new store is never cloneable by accident."""
+    from ledgrab.storage.base_sqlite_store import BaseSqliteStore
+
+    assert BaseSqliteStore._cloneable is False
+    assert ColorStripStore._cloneable is True
+    assert ValueSourceStore._cloneable is True
+
+
+def test_clone_refuses_non_cloneable_store(stores, monkeypatch):
+    """clone() refuses stores not on the allowlist (defence-in-depth even though
+    the duplicate endpoint already restricts to the safe kinds)."""
+    css, _vss = stores
+    src = css.create_source(name="X", source_type="single_color", colors=[[1, 1, 1]])
+    monkeypatch.setattr(type(css), "_cloneable", False)
+    with pytest.raises(NotImplementedError):
+        css.clone(src.id, "X (copy)")