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.

decnet/topology/config.py

@@ -59,6 +59,10 @@ class _PlannedLAN:
     subnet: str
     is_dmz: bool
     parent: Optional[str]  # name of parent LAN, None for DMZ
+    # Canvas coordinates — generator leaves them None; the web editor
+    # (or a future auto-layouter) fills them in.
+    x: Optional[float] = None
+    y: Optional[float] = None
 
 
 @dataclass
@@ -73,6 +77,9 @@ class _PlannedDecky:
     # Mirrors ``DeckyConfig.service_config`` from the flat-fleet path;
     # services read these via ``compose_fragment(service_cfg=...)``.
     service_config: dict[str, dict] = field(default_factory=dict)
+    # Canvas coordinates — see _PlannedLAN.x/y.
+    x: Optional[float] = None
+    y: Optional[float] = None
 
 
 @dataclass

decnet/topology/persistence.py

@@ -31,6 +31,8 @@ async def persist(repo: Any, plan: GeneratedTopology) -> str:
                 "name": lan.name,
                 "subnet": lan.subnet,
                 "is_dmz": lan.is_dmz,
+                "x": lan.x,
+                "y": lan.y,
             }
         )
         lan_ids[lan.name] = lan_id
@@ -55,6 +57,8 @@ async def persist(repo: Any, plan: GeneratedTopology) -> str:
                     "service_config": decky.service_config,
                 },
                 "ip": primary_ip,
+                "x": decky.x,
+                "y": decky.y,
             }
         )
         decky_ids[decky.name] = decky_uuid