Add catalog-zone-gen tool

Generate RFC 9432 DNS catalog zone files from a declarative input file.
Parses zone-to-catalog assignments with optional group and coo properties,
produces deterministic BIND-format output with automatic SOA serial
management and change detection.

go.mod

@@ -0,0 +1,14 @@
+module catalog-zone-gen
+
+go 1.25.5
+
+require (
+	codeberg.org/miekg/dns v0.6.61
+	gopkg.in/yaml.v3 v3.0.1
+)
+
+require (
+	golang.org/x/crypto v0.47.0 // indirect
+	golang.org/x/net v0.49.0 // indirect
+	golang.org/x/sys v0.40.0 // indirect
+)