feat(06-04): implement private/reserved IP classification table

- Port PHP PRIVATE_V4_CIDRS and PRIVATE_V6_PREFIXES onto netip.Prefix
- Fail closed on invalid addresses; Unmap remains the caller's job

fetchguard/ip.go

@@ -0,0 +1,45 @@
+package fetchguard
+
+import "net/netip"
+
+// privateV4 is a literal port of ManualCoverUrlFetcher.php PRIVATE_V4_CIDRS.
+var privateV4 = []netip.Prefix{
+	netip.MustParsePrefix("127.0.0.0/8"),
+	netip.MustParsePrefix("10.0.0.0/8"),
+	netip.MustParsePrefix("172.16.0.0/12"),
+	netip.MustParsePrefix("192.168.0.0/16"),
+	netip.MustParsePrefix("169.254.0.0/16"),
+	netip.MustParsePrefix("100.64.0.0/10"),
+	netip.MustParsePrefix("0.0.0.0/8"),
+}
+
+// privateV6 is a literal port of PRIVATE_V6_PREFIXES. PHP lists bare "::1"
+// as a prefix-less loopback literal; it is expressed here as ::1/128 so
+// Prefix.Contains works uniformly with the CIDR entries.
+var privateV6 = []netip.Prefix{
+	netip.MustParsePrefix("::1/128"),
+	netip.MustParsePrefix("fe80::/10"),
+	netip.MustParsePrefix("fc00::/7"),
+}
+
+// isReservedOrPrivate classifies addr against the PHP private/loopback/
+// reserved/CGNAT table. The caller must pass an already-Unmap()-ed address
+// (fetch.go's dial hook); this function does not Unmap.
+func isReservedOrPrivate(addr netip.Addr) bool {
+	if !addr.IsValid() {
+		return true
+	}
+	if addr.IsMulticast() || addr.IsUnspecified() {
+		return true
+	}
+	table := privateV4
+	if !addr.Is4() {
+		table = privateV6
+	}
+	for _, prefix := range table {
+		if prefix.Contains(addr) {
+			return true
+		}
+	}
+	return false
+}