feat: add NSD backend and Topology C (multi-instance with peer sync) 🏗️

- New NSDBackend: zone files + nsd-control reload, zone registration via
  nsd.conf.d include file; mirrors BIND backend interface exactly
- BackendRegistry now supports type "nsd"; config defaults for nsd.zones_dir
  and nsd.nsd_conf
- Dockerfile installs both NSD and BIND9 — entrypoint detects configured
  backend type(s) and starts only the required daemon; CoreDNS MySQL
  deployments start neither
- docker/nsd.conf: minimal NSD base config with remote-control and
  zones.conf include
- entrypoint.sh: reads config file + env vars to determine which daemon
  to start; runs nsd-control-setup on first boot
- 20 new NSD backend tests (117 total, all passing)
- README: Topology C (multi-instance + peer sync) documented as most robust
  HA option; NSD config reference; updated topology comparison table;
  NSD env-var-only compose examples; version 2.5.0

docker/nsd.conf

@@ -0,0 +1,20 @@
+# NSD base configuration for directdnsonly containers.
+# Zone stanzas are written to /etc/nsd/nsd.conf.d/zones.conf by the NSD
+# backend and auto-included via the glob below.
+
+server:
+    server-count: 1
+    ip-address: 0.0.0.0
+    port: 53
+    username: nsd
+    zonesdir: /etc/nsd/zones
+    verbosity: 1
+    # Log to stderr so Docker captures it
+    logfile: ""
+
+remote-control:
+    control-enable: yes
+    control-interface: 127.0.0.1
+    control-port: 8952
+
+include: /etc/nsd/nsd.conf.d/*.conf