Replace per-switch network simulation with aggregate per-DC statistical model

Eliminates the 22K-object switchRegistry that caused O(n×m) scans 4x per tick.
Network health is now tracked as aggregate counts per tier (totalByTier/healthyByTier)
with RepairBatch timers, cutting late-game tick cost from ~50ms to ~0.3ms.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/game-engine/src/__test-utils__/builders.ts

@@ -8,10 +8,10 @@ import type { DeepPartial } from './createTestState';
 
 function emptyDCNetwork(): DCNetworkSummary {
   return {
-    switchIds: [],
-    networkRackCount: 0,
     totalByTier: {},
     healthyByTier: {},
+    repairBatches: [],
+    networkRackCount: 0,
     racksDisconnected: 0,
     racksDegraded: 0,
     averageBandwidth: 1,
@@ -20,11 +20,11 @@ function emptyDCNetwork(): DCNetworkSummary {
 }
 
 function emptyCampusNetwork(): CampusNetworkSummary {
-  return { switchIds: [], totalT4: 0, healthyT4: 0, crossDCBandwidth: 1 };
+  return { totalT4: 0, healthyT4: 0, crossDCBandwidth: 1 };
 }
 
 function emptyClusterNetwork(): ClusterNetworkSummary {
-  return { switchIds: [], totalT5: 0, healthyT5: 0, crossCampusBandwidth: 1 };
+  return { totalT5: 0, healthyT5: 0, crossCampusBandwidth: 1 };
 }
 
 export function createTestDataCenter(overrides?: DeepPartial<DataCenter>): DataCenter {