fix: macOS install health check, harden recursive probe

Verify numa is listening (API port) before redirecting system DNS on
macOS — if the service fails to start (e.g. port 53 in use), unload
the service and abort instead of breaking DNS. Probe up to 3 root
hints before declaring recursive mode unavailable. Validate IPs from
resolvectl to avoid IPv6 fragment extraction. Extract DEFAULT_API_PORT
constant.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

README.md

@@ -24,7 +24,7 @@ sudo numa                              # port 53 requires root
 
 Open the dashboard: **http://numa.numa** (or `http://localhost:5380`)
 
-Set as system DNS: `sudo numa install && sudo numa service start`
+Set as system DNS: `sudo numa install`
 
 ## Local Services