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>

tooling/fetch-authlib.sh

@@ -0,0 +1,51 @@
+#!/usr/bin/env bash
+# fetch-authlib.sh — download the latest authlib-injector.jar into runtime/.
+#
+# The Minecraft container mounts ./runtime read-only at /extras and loads the
+# server JVM agent from /extras/authlib-injector.jar (points the server at Drasl
+# for session validation). The jar is gitignored, so a fresh checkout needs this.
+#
+# Usage:
+#   tooling/fetch-authlib.sh            # download if missing (or invalid)
+#   tooling/fetch-authlib.sh --force    # re-download even if present
+#
+# Cautious by design: resolves the release asset by name, halts on any error,
+# and verifies the result is a real zip/jar before trusting it (a half-written
+# or HTML error body would otherwise surface later as the cryptic
+# "Error opening zip file or JAR manifest missing").
+set -euo pipefail
+
+cd "$(dirname "$0")/.."   # repo root
+
+log()  { printf '\n\033[1;32m==>\033[0m %s\n' "$*"; }
+warn() { printf '\033[1;33m!!\033[0m %s\n' "$*" >&2; }
+die()  { printf '\033[1;31mxx\033[0m %s\n' "$*" >&2; exit 1; }
+
+command -v curl >/dev/null || die "curl required"
+command -v jq   >/dev/null || die "jq required"
+
+DEST="runtime/authlib-injector.jar"
+API="https://api.github.com/repos/yushijinhun/authlib-injector/releases/latest"
+
+# A valid jar is a zip — first two bytes are "PK". Reject anything else.
+is_jar() { [ -s "$1" ] && [ "$(head -c2 "$1" 2>/dev/null)" = "PK" ]; }
+
+if [ "${1:-}" != "--force" ] && is_jar "$DEST"; then
+  log "$DEST already present and valid — skipping (use --force to re-download)"
+  exit 0
+fi
+
+log "resolving latest authlib-injector release asset"
+url="$(curl -fsSL "$API" \
+  | jq -r '.assets[] | select(.name|test("authlib-injector-[0-9].*\\.jar$")) | .browser_download_url' \
+  | head -1)"
+[ -n "$url" ] && [ "$url" != "null" ] || die "could not resolve authlib-injector release asset"
+
+mkdir -p runtime
+tmp="${DEST}.part"
+log "downloading $(basename "$url")"
+curl -fSL --retry 3 -o "$tmp" "$url"
+
+is_jar "$tmp" || { rm -f "$tmp"; die "downloaded file is not a valid jar (zip) — aborting"; }
+mv -f "$tmp" "$DEST"
+log "done → $DEST ($(wc -c <"$DEST") bytes)"