Refuse [::], 0.0.0.0, IPv6 multicast and documentation space (closes #341)

On the build host a connection to [::] reaches a listener on ::1, and one to 0.0.0.0 reaches 127.0.0.1, so a delivery target at either reached this host's loopback past the guard. 0.0.0.0/32 and ::/128 are now in alwaysBlockedNetworks, which no allowlist opens; an allowlist reaches loopback only through an entry covering a loopback address. IPv6 multicast (ff00::/8) and documentation space (2001:db8::/32) are refused by default and reopen when listed.

Every default blocklist entry has a one-line comment, each list is pinned on its own, and tests refuse each address at target creation and at delivery. The README and the rules above each list match.

Model: opus-5-5

internal/config/config.go

@@ -196,12 +196,13 @@ type Config struct {
 	// otherwise refuse. The guard itself is always on: there is no
 	// setting that disables SSRF protection, and delivery's
 	// alwaysBlockedNetworks stays blocked no matter what is listed
-	// here. That set is link-local plus the cloud metadata
-	// endpoints outside it that disclose credentials or user data
-	// at a provider-fixed, non-public address; it is not
-	// exhaustive of every cloud's metadata address. See
-	// alwaysBlockedNetworks for the authoritative list and the
-	// criterion it is built from.
+	// here. That set is link-local, the unspecified addresses
+	// 0.0.0.0 and ::, and the cloud metadata endpoints outside
+	// link-local that disclose credentials or user data at a
+	// provider-fixed, non-public address; it is not exhaustive of
+	// every cloud's metadata address. See
+	// alwaysBlockedNetworks for the authoritative list and why
+	// each entry is on it.
 	AllowedEgressCIDRs []netip.Prefix
 
 	params *ConfigParams