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/LayoutSolverTests.swift

@@ -0,0 +1,45 @@
+import XCTest
+import CoreGraphics
+@testable import NetworkCityCore
+
+final class LayoutSolverTests: XCTestCase {
+
+    // A label box centered under the node: ~160 wide, sitting below the dot.
+    private let label = CGRect(x: -80, y: -34, width: 160, height: 30)
+
+    private func worldBox(at c: CGPoint) -> CGRect { label.offsetBy(dx: c.x, dy: c.y) }
+
+    func testEmptyMapReturnsSeed() {
+        let p = LayoutSolver.placeNonOverlapping(baseAngle: 0, baseRadius: 200, localRect: label, existing: [])
+        XCTAssertEqual(p.x, 200, accuracy: 0.001)   // attempt 0 == seed
+        XCTAssertEqual(p.y, 0, accuracy: 0.001)
+    }
+
+    func testAvoidsOverlapWithExisting() {
+        // Pre-place a box exactly where the seed would land.
+        let seed = CGPoint(x: cos(0.0) * 200, y: sin(0.0) * 200)
+        let existing = [worldBox(at: seed).insetBy(dx: -16, dy: -16)]
+
+        let p = LayoutSolver.placeNonOverlapping(baseAngle: 0, baseRadius: 200, localRect: label, existing: existing)
+        XCTAssertNotEqual(p, seed)   // had to move
+
+        // The chosen spot's padded box must clear the existing one.
+        let placed = worldBox(at: p).insetBy(dx: -16, dy: -16)
+        XCTAssertFalse(placed.intersects(existing[0]))
+    }
+
+    func testResultClearsAllOfManyNeighbours() {
+        // Seed eight districts and confirm none of their boxes overlap.
+        var placed: [CGRect] = []
+        for i in 0..<8 {
+            let angle = Double(i) / 8 * 2 * .pi
+            let c = LayoutSolver.placeNonOverlapping(baseAngle: angle, baseRadius: 180, localRect: label, existing: placed)
+            placed.append(worldBox(at: c))
+        }
+        for a in 0..<placed.count {
+            for b in (a + 1)..<placed.count {
+                XCTAssertFalse(placed[a].intersects(placed[b]), "districts \(a) and \(b) overlap")
+            }
+        }
+    }
+}