feat(tooling): fetch-authlib.sh to download authlib-injector.jar

The server JVM agent (runtime/authlib-injector.jar, mounted at /extras) is
gitignored and was previously fetched by the deleted build-stack.sh prep. A
fresh checkout had no jar, so minecraft crashlooped with "Error opening zip
file or JAR manifest missing". Add a standalone fetch tool: resolves the latest
release asset, downloads, and verifies it's a real zip/jar (PK magic) before
trusting it. Idempotent (skips if valid, --force to re-download). Wire it into
the deploy skill and README launch steps.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

README.md

@@ -49,7 +49,7 @@ cp .env.example .env            # set BASE_DOMAIN, RCON_PASSWORD, DISTRIBUTION_W
 tooling/render-config.sh        # drasl + nmsr + packwiz configs from *.tmpl
 ( cd landing && pnpm install && BASE_DOMAIN="$BASE_DOMAIN" pnpm run build )   # → www/
 tooling/fetch-launcher.sh       # FjordLauncher assets → launcher/ (served at /launcher/)
-# authlib-injector.jar → runtime/ (server JVM agent; see drasl docs)
+tooling/fetch-authlib.sh        # authlib-injector.jar → runtime/ (server JVM agent)
 
 docker compose up -d --build    # first run installs NeoForge + mods, builds nmsr
 docker compose down             # stop