Fix ips_to_range using XOR instead of subtraction for prefix length

Subtraction underestimates the required prefix when IPs span a CIDR
boundary (e.g. .110–.119 gave /28 covering only .96–.111, leaving
deckies at .112+ unreachable from the host macvlan route).
XOR correctly finds the highest differing bit, yielding /27 (.96–.127).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

decnet/network.py

@@ -208,7 +208,7 @@ def ips_to_range(ips: list[str]) -> str:
     """
     addrs = [IPv4Address(ip) for ip in ips]
     network = IPv4Network(
-        (int(min(addrs)), 32 - (int(max(addrs)) - int(min(addrs))).bit_length()),
+        (int(min(addrs)), 32 - (int(max(addrs)) ^ int(min(addrs))).bit_length()),
         strict=False,
     )
     return str(network)