fix: create bench/ dir before opening server log (fresh-checkout crash)

Server.__enter__ opened the log file before mkdir -p bench, so the first run on a
clean checkout (bench/ is gitignored) crashed with FileNotFoundError. Reorder.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: Leonardo

selfdraft/sd.py

@@ -68,8 +68,8 @@ def cmd(self):
         return c
 
     def __enter__(self):
-        self.log = open(os.path.join(ROOT, "bench", f"server_{self.port}.log"), "w")
         os.makedirs(os.path.join(ROOT, "bench"), exist_ok=True)
+        self.log = open(os.path.join(ROOT, "bench", f"server_{self.port}.log"), "w")
         self.proc = subprocess.Popen(self.cmd(), stdout=self.log, stderr=subprocess.STDOUT,
                                      preexec_fn=os.setsid)
         # wait for /health