fix(mazenet): auto-layout nets + deckies in a deterministic grid

Dropping more than one LAN near the same spot stacked the NetBox
rectangles on top of each other, and multiple deckies in a LAN
landed on identical per-LAN coordinates. Since canvas position
persistence is deferred (localStorage pass), the stored x/y are
not load-bearing — compute layout from the topology graph instead.

adaptTopology now lays LANs out in a 3-col grid with the DMZ first
and stacks deckies 2-wide inside their home LAN. New LAN palette
drops append to the same grid, ignoring the raw drop point.

decnet_web/src/components/MazeNET/MazeNET.tsx

@@ -67,9 +67,14 @@ const MazeNET: React.FC = () => {
           flashErr(null, 'topology already has a DMZ');
           return;
         }
-        const w = 320, h = 240;
-        const x = Math.round(world.x - w / 2);
-        const y = Math.round(world.y - h / 2);
+        // Append to the 3-col grid matching adaptTopology so new drops
+        // never land on top of existing LANs. The raw drop point is
+        // ignored — cleaner than trying to resolve collisions after.
+        const w = 300, h = 240;
+        const GAP = 40, COLS = 3;
+        const i = nets.filter((n) => n.kind !== 'internet').length;
+        const x = GAP + (i % COLS) * (w + GAP);
+        const y = GAP + Math.floor(i / COLS) * (h + GAP);
         const name = isDmz ? `dmz-${hex4()}` : `subnet-${hex4()}`;
         try {
           const subnet = await api.getNextSubnet().catch(() => undefined);