Zone matrix: what the rules do between zones

internal/matrix walks each pair's rules in order, as nftables would, for
new connections: zone → zone (forward), zone → the firewall (input) and
back (output), with the built-ins that act on new connections. A rule
for the whole pair decides it; one for part of its traffic (addresses,
services, times, rates, a quota) is an exception; the rest falls to the
policy. Rules whose addresses all lie in networks connected to other
zones are left out of a pair. GET /matrix (with ?changeset, pending
changes), hc matrix [FROM TO], and a Matrix page whose cells open their
rules.

e2e: 166 checks, among them the matrix of the maintainer's imported
ruleset.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -48,7 +48,8 @@ table of its own.
   ruleset into zones, sets, services, rules and port forwards, keeping the
   rules' order, and keeps what it can't translate as raw rules
   ([docs/import.md](docs/import.md)).
-* **Easy to watch.** Hits per rule, a live log of what was dropped and
+* **Easy to watch.** A zone matrix of what is allowed between zones,
+  hits per rule, a live log of what was dropped and
   why, the connections the firewall tracks (and closing them), a packet
   tracer that names the rule that dropped something, in any table, and
   the history of every change, field by field, with undo.