Add idle district lifecycle: decay then reap

The map only ever gained density: hubs were sized by lifetime-cumulative
bytes (so they never shrank) and were never removed. Now each hub has a
decaying activity level — quiet districts visibly shrink toward a dormant
dot — and after a configurable idle timeout (~40s, NC_REAP_TICKS) the hub
is demolished: faded out, removed with its road, and its per-org state
pruned in the controller.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Sources/NetworkCityApp/TrafficController.swift

@@ -39,6 +39,9 @@ final class TrafficController: ObservableObject {
         scene.onSelect = { [weak self] key in
             MainActor.assumeIsolated { self?.select(key) }
         }
+        scene.onReap = { [weak self] key in
+            MainActor.assumeIsolated { self?.prune(key) }
+        }
         if ProcessInfo.processInfo.environment["NC_DEMO"] != nil {
             startDemo()
             return
@@ -175,6 +178,17 @@ final class TrafficController: ObservableObject {
 
     func deselect() { select(nil) }
 
+    /// A district was demolished — drop its rendering/inspection state. The
+    /// resolution + RTT caches are kept (small, bounded by distinct hosts, and
+    /// save rework if the district comes back).
+    private func prune(_ key: String) {
+        orgEndpoints[key] = nil
+        orgProcesses[key] = nil
+        orgTitles[key] = nil
+        lastEndpointBytes[key] = nil
+        if selectedKey == key { selectedKey = nil; selection = nil }
+    }
+
     private func buildInspection(_ key: String) -> OrgInspection {
         let hosts = orgEndpoints[key] ?? []
         var infos: [EndpointInfo] = []