Initial commit: NetworkCity — network traffic as a neon city

A native macOS network monitor that renders live traffic as a top-down
neon city: your Mac is downtown, remote orgs are glowing districts, and
traffic is cars of light driving the roads.

Architecture:
- NetworkCityCore: swappable data layer behind a ConnectionSource protocol
  (nettop-backed today), plus pure/tested logic — traffic diffing, org
  classification (reverse-DNS + CIDR), traffic classes, hub-and-spoke
  expansion policy, label anti-overlap, and latency→distance layout.
- NetworkCityApp: SwiftUI + SpriteKit city — auto-expanding districts,
  protocol-coloured cars, click-to-inspect panel, and latency-as-distance
  (districts glide to their measured RTT).
- nettop-probe: CLI proof of the data layer.

44 tests passing.

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

Tests/NetworkCityCoreTests/LatencyLayoutTests.swift

@@ -0,0 +1,35 @@
+import XCTest
+import CoreGraphics
+@testable import NetworkCityCore
+
+final class LatencyLayoutTests: XCTestCase {
+
+    func testZeroRTTSitsAtInnerRadius() {
+        XCTAssertEqual(LatencyLayout.radius(forRTT: 0), LatencyLayout.minRadius, accuracy: 0.001)
+    }
+
+    func testCeilingSitsAtOuterEdge() {
+        XCTAssertEqual(LatencyLayout.radius(forRTT: 250), LatencyLayout.maxRadius, accuracy: 0.001)
+    }
+
+    func testClampsBeyondCeiling() {
+        XCTAssertEqual(LatencyLayout.radius(forRTT: 5000), LatencyLayout.maxRadius, accuracy: 0.001)
+    }
+
+    func testMonotonicAndWithinBounds() {
+        var last = LatencyLayout.radius(forRTT: 0)
+        for rtt in stride(from: 5.0, through: 250.0, by: 5) {
+            let r = LatencyLayout.radius(forRTT: rtt)
+            XCTAssertGreaterThan(r, last)   // farther RTT -> farther out
+            XCTAssertLessThanOrEqual(r, LatencyLayout.maxRadius)
+            last = r
+        }
+    }
+
+    func testNearRangeSpreadsOut() {
+        // The log curve should give the 1–50ms band real separation, not a clump.
+        let r10 = LatencyLayout.radius(forRTT: 10)
+        let r50 = LatencyLayout.radius(forRTT: 50)
+        XCTAssertGreaterThan(r50 - r10, 40)   // meaningfully far apart on screen
+    }
+}