Fix zone list filtering and BIND warning display; add a views guide

- jarl zone list showed only primary zones. zone create binds its --type
  flag (default "primary") to the same variable as list's --type, so the
  default leaked into the list filter and hid hint, secondary and forward
  zones. list now has its own variable. TestListQueries runs list
  commands against a fake server and checks the filters they send.
- named-checkconf/named-checkzone messages are classified by the tool's
  exit code: on success, everything printed is a warning. Warnings such
  as 'both "recursion no;" and "allow-recursion" active' were shown as
  errors. The CLI, jarld check and the web UI now label them as warnings
  and skip the routine "loaded serial" lines.
- The zone_outside_view error points at --adopt-zones.
- docs/views.md: step-by-step split DNS for a server that has no views
  yet, tested on a copy of a stock Ubuntu 26.04 setup. Linked from the
  README, concepts and validation docs.

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

internal/bind/bind_test.go

@@ -0,0 +1,19 @@
+package bind
+
+import "testing"
+
+func TestResultClassification(t *testing.T) {
+	// named-checkconf exited 0: everything it printed is a warning.
+	ok := newResult(`both "recursion no;" and "allow-recursion" active for view external
+zone example.com/IN/internal: loaded serial 1
+`, 0)
+	if !ok.OK || len(ok.Messages) != 2 || ok.Messages[0].Error || ok.Messages[1].Error {
+		t.Errorf("exit 0: %+v", ok)
+	}
+	// Non-zero exit: real problems stay errors.
+	bad := newResult(`/etc/bind/jarl/named.conf:12: unknown option 'recursoin'
+`, 1)
+	if bad.OK || len(bad.Messages) != 1 || !bad.Messages[0].Error || bad.Messages[0].Line != 12 {
+		t.Errorf("exit 1: %+v", bad)
+	}
+}