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.

testdata/zones.txt

@@ -0,0 +1,5 @@
+# Production zones
+zone.example.org    catalog1, catalog2
+zone.example.com    catalog2, coo=old-catalog.example.com.
+test.example.net    catalog1, group=internal
+app.example.org     catalog1, group=external, coo=migrated.example.com.