Pull progress: 3s snapshot throttle

Cold pulls now tick a 'Downloading xMB/yGB' snapshot at most every 3s
(was 10s) -- keyword hits (Pull complete / Status / errors) still
print immediately.

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

evalharness/sandbox/prefetch.py

@@ -123,8 +123,8 @@ def _pull_one(image: str, verbose: bool = True) -> None:
             now = _time.monotonic()
             if verbose and (any(k in line for k in
                                 ('Pulling from', 'Pull complete', 'Status',
-                                 'error', 'denied'))
-                            or now - last_show > 10):
+                                 'error', 'denied', 'Extracting', 'Verifying'))
+                            or now - last_show > 3):   # Downloading snapshots
                 print(f'  {last_line[:100]}', file=_sys.stderr, flush=True)
                 last_show = now
         proc.wait(timeout=3600)