feat(topology): nullable layout coords on LAN + TopologyDecky

MazeNET phase 2 step 5. Pure storage — the generator emits None for
x/y and the web canvas fills them in later. No logic changes; no
compose, deploy, or validator impact.

tests/topology/test_layout.py

@@ -0,0 +1,58 @@
+"""Layout coordinate roundtrips for LAN and TopologyDecky."""
+from __future__ import annotations
+
+import pytest
+
+from decnet.topology.config import TopologyConfig
+from decnet.topology.generator import generate
+from decnet.topology.persistence import hydrate, persist
+from decnet.web.db.factory import get_repository
+
+
+def _cfg(**kw) -> TopologyConfig:
+    base = dict(
+        name="layout",
+        depth=1,
+        branching_factor=1,
+        deckies_per_lan_min=1,
+        deckies_per_lan_max=1,
+        cross_edge_probability=0.0,
+        randomize_services=False,
+        services_explicit=["ssh"],
+        seed=4,
+    )
+    base.update(kw)
+    return TopologyConfig(**base)
+
+
+@pytest.fixture
+async def repo(tmp_path):
+    r = get_repository(db_path=str(tmp_path / "layout.db"))
+    await r.initialize()
+    return r
+
+
+@pytest.mark.anyio
+async def test_coords_roundtrip_when_set(repo):
+    plan = generate(_cfg())
+    plan.lans[0].x = 10.5
+    plan.lans[0].y = -3.25
+    plan.deckies[0].x = 42.0
+    plan.deckies[0].y = 7.5
+    tid = await persist(repo, plan)
+    hydrated = await hydrate(repo, tid)
+    lan = next(l for l in hydrated["lans"] if l["name"] == plan.lans[0].name)
+    assert lan["x"] == 10.5 and lan["y"] == -3.25
+    d = next(d for d in hydrated["deckies"] if d["name"] == plan.deckies[0].name)
+    assert d["x"] == 42.0 and d["y"] == 7.5
+
+
+@pytest.mark.anyio
+async def test_coords_default_to_none(repo):
+    plan = generate(_cfg())
+    tid = await persist(repo, plan)
+    hydrated = await hydrate(repo, tid)
+    for lan in hydrated["lans"]:
+        assert lan["x"] is None and lan["y"] is None
+    for d in hydrated["deckies"]:
+        assert d["x"] is None and d["y"] is None