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/BuildingNode.swift

@@ -39,7 +39,7 @@ final class BuildingNode: SKNode {
     private let core: SKShapeNode
     private let title: SKLabelNode
     private let subtitle: SKLabelNode
-    private var cumulative: Double = 0
+    private var activity: Double = 0
 
     private let spokeLayer = SKNode()
     private let endpointLayer = SKNode()
@@ -106,13 +106,11 @@ final class BuildingNode: SKNode {
 
     func note(activeHosts: [String]) { knownHosts.formUnion(activeHosts) }
 
-    /// React to a tick of traffic: grow toward a size set by total volume, and
-    /// pulse so the eye catches the activity.
+    /// React to a tick of traffic: bump the (decaying) activity level, grow
+    /// toward a size set by it, and pulse so the eye catches the activity.
     func receive(bytes: Double) {
-        cumulative += bytes
-        let target = (0.30 + CGFloat(log10(cumulative + 1)) * 0.09).clamped(0.30, 1.05)
-        glow.removeAction(forKey: "resize")
-        glow.run(.scale(to: target, duration: 0.6), withKey: "resize")
+        activity += bytes
+        updateGlowSize()
 
         core.removeAction(forKey: "pulse")
         core.run(.sequence([
@@ -121,6 +119,23 @@ final class BuildingNode: SKNode {
         ]), withKey: "pulse")
     }
 
+    /// Called every tick. Activity decays so quiet hubs visibly shrink toward
+    /// dormant instead of staying bright forever.
+    func decay(factor: Double = 0.55) {
+        guard activity > 0 else { return }
+        activity *= factor
+        updateGlowSize()
+    }
+
+    /// Ticks since this hub last carried traffic — used to reap dead districts.
+    var idleTicks = 0
+
+    private func updateGlowSize() {
+        let target = (0.20 + CGFloat(log10(activity + 1)) * 0.085).clamped(0.20, 1.05)
+        glow.removeAction(forKey: "resize")
+        glow.run(.scale(to: target, duration: 0.6), withKey: "resize")
+    }
+
     // MARK: - Expansion
 
     /// The capped, stably-ordered list of endpoints we actually draw.