chore: checkpoint stable battle and eidolon progression

internal/common/persistent_lock_unix.go

@@ -0,0 +1,20 @@
+//go:build linux || darwin || freebsd || openbsd || netbsd || dragonfly
+
+package common
+
+import (
+	"os"
+	"syscall"
+)
+
+func lockPersistentFile() (func(), error) {
+	file, err := os.OpenFile("persistent.lock", os.O_CREATE|os.O_RDWR, 0600)
+	if err != nil {
+		return nil, err
+	}
+	if err := syscall.Flock(int(file.Fd()), syscall.LOCK_EX); err != nil {
+		file.Close()
+		return nil, err
+	}
+	return func() { syscall.Flock(int(file.Fd()), syscall.LOCK_UN); file.Close() }, nil
+}