watcher: an IPv6 address is written in brackets before its port (closes #270)

Port and certificate keys were joined as address:port, so an IPv6
address ran into its port: 2a01:4f8:2200:fd11::1:80 is also an address
on its own. Keys are now joined with net.JoinHostPort and split with
net.SplitHostPort, giving [2a01:4f8:2200:fd11::1]:80 in the state file,
/api/v1/status, the dashboard and Port Change notifications; an IPv4
address is unchanged. A certificate key still splits at its last colon,
since a hostname has none. Tests that built IPv6 keys by hand now use
the bracketed form, and a test checks that an IPv6 address's
certificate entry is removed when its port 443 closes.

Model: opus-5-5

TODO.md

@@ -19,6 +19,8 @@ milestone: https://git.eeqj.de/sneak/dnswatcher/milestone/7
 
 # Completed Steps
 
+- 2026-10-13: an IPv6 address is written in brackets before its port, as in
+  `[2001:db8::1]:443`, in keys and Port Change notifications (closes #270).
 - 2026-10-13: `TODO.md` names only open issues as next and future steps, and its
   Status says nothing about git tags (closes #281).
 - 2026-10-13: a port check that finds port 443 closed removes that address's