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>

docker-compose.static.yml

@@ -8,3 +8,4 @@ services:
       - ./caddy/conf.d/10-static.caddy:/etc/caddy/conf.d/10-static.caddy:ro
       - ./www:/srv/www:ro
       - ./mirror/launcher:/srv/launcher:ro
+      - ./caddy/caddy-root-ca.crt:/srv/ca-pub/ca.crt:ro