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

On Linux a connection to the unspecified address [::] or 0.0.0.0
reaches the host's own loopback, and the SSRF guard let [::] through.
Both unspecified addresses now sit in alwaysBlockedNetworks, so an
allowlist reaches loopback only through an entry that covers a
loopback address, never through one that covers only 0.0.0.0 or ::;
::/128 joins the default blocklist beside 0.0.0.0/8. IPv6 multicast
(ff00::/8) and documentation space (2001:db8::/32) are refused by
default. Every default blocklist entry gets a one-line comment, and the
README, the rules above each list, the two pinning tests and the
allowlist refusal test follow.

Model: opus-5-5

internal/config/config.go

@@ -196,10 +196,11 @@ 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
+	// 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 the
 	// criterion it is built from.
 	AllowedEgressCIDRs []netip.Prefix