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>

Sources/NetworkCityCore/ConnectionSource.swift

@@ -0,0 +1,12 @@
+import Foundation
+
+/// The seam between *where the bytes come from* and *the city that renders them*.
+///
+/// Phase 1 backs this with `NettopSource`. Phase 2 can swap in a libpcap-based
+/// source — anything that can emit `ConnectionSnapshot`s — and the renderer is
+/// none the wiser.
+public protocol ConnectionSource: Sendable {
+    /// Emits a snapshot roughly every `interval` seconds until the stream is
+    /// cancelled (e.g. the consumer breaks out of its `for await` loop).
+    func snapshots(every interval: TimeInterval) -> AsyncStream<ConnectionSnapshot>
+}