refactor(caddy): split vhosts into conf.d snippets (core/static/mirror)

Base Caddyfile now imports conf.d/*.caddy. Snippets are mounted selectively
so the stack runs with or without the static site / air-gap mirror:
  00-core   auth + packwiz (always)
  10-static apex landing + launcher downloads
  20-mirror air-gap upstream hosts (tls internal)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

caddy/conf.d/00-core.caddy

@@ -0,0 +1,9 @@
+# Core ingress — always mounted. Drasl auth + packwiz pack metadata.
+http://auth.{$BASE_DOMAIN} {
+	reverse_proxy drasl:25585
+}
+
+http://packwiz.{$BASE_DOMAIN} {
+	root * /srv/pack
+	file_server browse
+}