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 by naming it; ::/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 and the two pinning tests follow.

Model: opus-5-5

README.md

@@ -158,19 +158,20 @@ WireServer, which serves an Azure VM its credentials. Because it is a
 public address, listing it in `ALLOWED_EGRESS_CIDRS` reopens it.
 
 That is all the default blocklist covers: the IPv4 private and reserved
-ranges; of IPv6, only loopback (`::1`), unique local addresses
-(`fc00::/7`) and link-local addresses (`fe80::/10`); and certain public
-addresses. A public address belongs on the default blocklist only if it
-hands credentials, user data or bootstrap material to whatever can reach
-it, without the caller presenting anything. A provider's other public
-addresses are not refused. IBM Cloud, for example, serves its package
-mirrors, time servers and object storage on `161.26.0.0/16`, and the
-private endpoints of its own cloud services on `166.8.0.0/14`. Neither
-range hands out credentials that way: the token service among those
-endpoints issues a token only in exchange for something the caller
-presents, such as an API key. Reaching these services can be a
-legitimate delivery, and every cloud has some, so a partial list would
-promise coverage it does not give.
+ranges; of IPv6, only loopback (`::1`), the unspecified address (`::`),
+unique local addresses (`fc00::/7`), link-local addresses (`fe80::/10`),
+multicast (`ff00::/8`) and documentation space (`2001:db8::/32`); and
+certain public addresses. A public address belongs on the default
+blocklist only if it hands credentials, user data or bootstrap material
+to whatever can reach it, without the caller presenting anything. A
+provider's other public addresses are not refused. IBM Cloud, for
+example, serves its package mirrors, time servers and object storage on
+`161.26.0.0/16`, and the private endpoints of its own cloud services on
+`166.8.0.0/14`. Neither range hands out credentials that way: the token
+service among those endpoints issues a token only in exchange for
+something the caller presents, such as an API key. Reaching these
+services can be a legitimate delivery, and every cloud has some, so a
+partial list would promise coverage it does not give.
 
 That default is also inconvenient for the thing webhooker is mostly
 for: taking a public webhook and forwarding it to something on your own
@@ -210,16 +211,16 @@ Two things this setting cannot do:
   the list is always an allowlist; an empty list (the default) means
   every private and reserved range stays refused. Note that
   `0.0.0.0/0` gets you most of the way there anyway, per above.
-- **It cannot open link-local, or a cloud metadata endpoint at a
-  non-public address that discloses credentials or user data.** An
-  address is on the list below when it is not a public address and both
-  of these hold: the provider fixes it, so it cannot collide with
-  anything you run; and reaching it hands out credentials, user data or
-  bootstrap material. Those stay blocked no matter what you list,
-  including when you list them outright or list a supernet such as
-  `0.0.0.0/0`, `::/0`, `fd00::/8` or `100.64.0.0/10`. Treat this as best
-  effort rather than a guarantee — it is a hand-maintained list and the
-  caveat below the table applies:
+- **It cannot open link-local, the unspecified addresses, or a cloud
+  metadata endpoint at a non-public address that discloses credentials
+  or user data.** A metadata address is on the list below when it is not
+  a public address and both of these hold: the provider fixes it, so it
+  cannot collide with anything you run; and reaching it hands out
+  credentials, user data or bootstrap material. Those stay blocked no
+  matter what you list, including when you list them outright or list a
+  supernet such as `0.0.0.0/0`, `::/0`, `fd00::/8` or `100.64.0.0/10`.
+  Treat this as best effort rather than a guarantee — it is a
+  hand-maintained list and the caveat below the table applies:
 
   | Blocked unconditionally | What it is |
   | ----------------------- | ---------- |
@@ -233,14 +234,22 @@ Two things this setting cannot do:
   | `fd00:a9fe:a9fe::1/128` | Linode/Akamai metadata over IPv6 |
   | `100.100.100.200/32` | Alibaba Cloud metadata, inside CGNAT |
   | `192.0.0.192/32` | Oracle Cloud Classic metadata |
+  | `0.0.0.0/32` | IPv4 unspecified address, which reaches this host's loopback on Linux |
+  | `::/128` | IPv6 unspecified address, which reaches this host's loopback on Linux |
   | `::a9fe:a9fe/128` | `169.254.169.254` as an IPv4-compatible IPv6 address |
   | `64:ff9b::a9fe:a9fe/128` | `169.254.169.254` behind the NAT64 well-known prefix |
 
   The IPv4-mapped form `::ffff:169.254.169.254` is covered by the
-  `169.254.0.0/16` entry. Reaching any of these is credential or
-  user-data theft rather than delivery to an internal service. Every
-  entry outside the two link-local blocks is a single address, so
-  blocking it costs you nothing else on the network around it.
+  `169.254.0.0/16` entry. Reaching any of these but the two unspecified
+  addresses is credential or user-data theft rather than delivery to an
+  internal service. Every entry outside the two link-local blocks is a
+  single address, so blocking it costs you nothing else on the network
+  around it.
+
+  The unspecified addresses `0.0.0.0` and `::` hand out nothing
+  themselves, but no host can have either, and on Linux a connection to
+  one reaches this host's own loopback. They are listed so that the only
+  way to open loopback is to name it, as `127.0.0.0/8` or `::1`.
 
   The six ULA entries, all inside `fd00::/8`, are why this matters in
   practice: `fd00::/8` is an ordinary block to allowlist for your own
@@ -3093,7 +3102,8 @@ check, see [The login endpoint](#the-login-endpoint).
   route through a single decision function, so they cannot disagree
   about a destination. An operator can permit specific blocks with
   [`ALLOWED_EGRESS_CIDRS`](#allowing-egress-to-your-own-network); the
-  guard cannot be switched off, and link-local plus a
+  guard cannot be switched off, and link-local, the unspecified
+  addresses `0.0.0.0` and `::`, and a
   [pinned set](#allowing-egress-to-your-own-network) of known cloud
   metadata endpoints — several of which are ULAs outside link-local —
   stay blocked whatever is listed, though listing `0.0.0.0/0` or