feat: discover printer by MAC (survives DHCP IP changes)

Add discover_printer(), _mac_of(), _port_open(), _sweep_arp(), _cache_ip()
with PRINTER_MAC/PRINTER_SUBNET/CACHE_FILE config + unit tests. Also adds
conftest.py to work around broken symlink that blocked pytest collection.

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

config.example.json

@@ -12,5 +12,7 @@
     "remote_host": "192.168.1.x",
     "remote_pass": "ssh-password",
     "printer_ip": "192.168.1.x",
-    "printer_port": 9100
+    "printer_port": 9100,
+    "printer_mac": "aa:bb:cc:dd:ee:ff",
+    "printer_subnet": "192.168.1"
 }