v1.7.0 — macOS support (experimental: runtime untested on real hardware)

WHAT
- SystemProxy via the native SystemConfiguration framework: per-service
  Proxies dictionaries are patched with a merge that preserves unrelated
  user settings (SOCKS, exclusions). Manual HTTP/HTTPS proxy and PAC
  auto-config URL both supported; clean removal on shutdown.
- Autostart as a launchd user agent: ~/Library/LaunchAgents/com.janus.proxy.plist
  (RunAtLoad + KeepAlive), registered via launchctl bootstrap gui/<uid>.
- Tray icon on the AppKit main thread: the async engine moves to a side
  thread, the main thread owns a CFRunLoop serving an NSStatusItem built
  with tray-icon; menu clicks flow into the same TrayCommand channel.
- Makefile targets for macOS builds on a Mac or CI runner:
  build-darwin-arm64 / build-darwin-x86_64 / build-darwin-universal (lipo).
- rustls/tokio-rustls/axum-server pinned to the ring crypto provider:
  single-provider process, and aws-lc-sys's C build no longer blocks
  darwin cross-checks.

STATUS
- Compile-verified from Linux: cargo check --target aarch64-apple-darwin.
- NOT yet exercised at runtime on real macOS hardware — proxy registration,
  launchd agent and the tray need a smoke test on a Mac or a CI macos runner
  before this platform can be called stable.