fix: drop nested save_file import so the closure can find it on cache-hit

The redundant `from safetensors.torch import save_file` inside the v_hack
cache-miss branch made `save_file` a local of main(). Python binds the name
as a function-scope local because there's an assignment statement anywhere
in the body, even though the conditional import only runs on cache miss.
The top-level import at line 75 was shadowed for the whole function.

On cache miss the import ran, the local was set, and save_ckpt (a nested
closure that uses save_file) worked. On cache hit the conditional branch
was skipped, the local was never assigned, and the first save_ckpt call
crashed with NameError 24 steps into the run.

#54 hit this. #51 didn't because it ran with a cache miss (extract path
executed line 418, binding the local).

src/projected_grpo/train.py

@@ -415,7 +415,6 @@ def main(cfg: Config) -> int:
     if not v_hack_path.exists():
         from .extract_vhack_grad import extract_v_hack
         from .pairs import PAIRS as VHACK_PAIRS
-        from safetensors.torch import save_file
         logger.info(f"v_hack cache miss at {v_hack_path}; extracting (~5min)...")
         model.eval()  # match standalone extract: deterministic backward, no dropout
         v_hack_cpu_dict, raw_grads, _diag = extract_v_hack(