feat(ipv4): require numeric prefix in strict CIDR mode

HU-04: Enforce strict CIDR notation in strict mode by requiring the
prefix to be a numeric value (0-32) instead of accepting dotted netmask
notation like 255.255.255.0.

When strict=True with CIDR enabled:
- Accepts: 192.168.1.0/24
- Rejects: 192.168.1.0/255.255.255.0

This change maintains backward compatibility:
- Non-strict mode continues to accept both formats
- All existing tests continue to pass
- Only adds stricter validation when explicitly requested

Author: ericksc

src/validators/ip_address.py

@@ -88,6 +88,10 @@ def ipv4(
         if cidr:
             if strict and value.count("/") != 1:
                 raise ValueError("IPv4 address was expected in CIDR notation")
+            if strict:
+                _, prefix = value.split("/")
+                if not prefix.isdigit() or not (0 <= int(prefix) <= 32):
+                    raise ValueError("CIDR prefix must be a number between 0 and 32")
             return IPv4Network(value, strict=not host_bit) and _check_private_ip(value, private)
         return IPv4Address(value) and _check_private_ip(value, private)
     except (ValueError, AddressValueError, NetmaskValueError):