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>

cmd/hc/main.go

@@ -191,7 +191,7 @@ Exit codes: 0 ok, 1 error, 3 not authenticated or not allowed, 4 not found,
 		}
 	}
 	add("fw", statusCmd(), zoneCmd(), setCmd(), serviceCmd(), ruleCmd(), forwardCmd(), snatCmd(), redirectCmd(), settingsCmd(),
-		rulesetCmd(), countersCmd(), quotasCmd(), logCmd(), connectionsCmd(), traceCmd(), interfaceCmd())
+		rulesetCmd(), countersCmd(), quotasCmd(), logCmd(), connectionsCmd(), traceCmd(), matrixCmd(), interfaceCmd())
 	add("changes", changesetCmd(), confirmCmd(), rollbackCmd(), historyCmd(), importCmd(), takeoverCmd())
 	add("admin", whoamiCmd(), userCmd(), tokenCmd(), passwordCmd(), certCmd(), auditCmd(), configCmd())
 	return root