feat(caddy): serve Caddy root CA at /ca.crt for guest trust

Air-gap mirror uses tls internal; guests must trust Caddy's local CA. Serve it
at http://<domain>/ca.crt from a world-readable mount (the live pki dir is
root-only 0600). build-stack --up mirror|full exports the CA after bring-up;
landing page links it. Pre-creates the mount target to avoid a dir bind.

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

caddy/conf.d/10-static.caddy

@@ -1,6 +1,13 @@
 # Static toggle — apex landing page + launcher downloads.
 # Mounted only when the stack runs with static files (build-stack.sh up static|full).
 http://{$BASE_DOMAIN} {
+	# Caddy's local CA root, so guests can trust the air-gap HTTPS mirror.
+	# Mounted from ./caddy/caddy-root-ca.crt (exported by build-stack prep;
+	# the live pki dir is root-only 0600 and can't be served directly).
+	handle /ca.crt {
+		root * /srv/ca-pub
+		file_server
+	}
 	# Launcher downloads are a sibling mount (/srv/launcher), not nested under
 	# the read-only /srv/www. handle_path strips the /launcher prefix.
 	handle_path /launcher/* {