Complete 26.3 parity acceptance and dual-version release workflow

scripts/server_test_lock.py

@@ -0,0 +1,18 @@
+"""One managed temporary Minecraft server at a time on this host."""
+from contextlib import contextmanager
+import fcntl
+from pathlib import Path
+import tempfile
+
+
+@contextmanager
+def server_test_lock():
+    with (Path(tempfile.gettempdir()) / "rivencraft-server-tests.lock").open("a") as lock:
+        try:
+            fcntl.flock(lock, fcntl.LOCK_EX | fcntl.LOCK_NB)
+        except BlockingIOError:
+            raise SystemExit("Another Rivencraft server test is active; finish it before starting this one")
+        try:
+            yield
+        finally:
+            fcntl.flock(lock, fcntl.LOCK_UN)