fix(simple): report missing weights file cleanly instead of traceback in verify #294

Description

In simple/ovl.py, passing a nonexistent file path to --weights in verify causes load_weights to crash with an unhandled FileNotFoundError traceback:

Traceback (most recent call last):
  File "simple/ovl.py", line 620, in cmd_verify
    sd = load_weights(a.weights)
  File "simple/ovl.py", line 544, in load_weights
    return torch.load(path, map_location="cpu", weights_only=True)
FileNotFoundError: [Errno 2] No such file or directory: 'nonexistent.pt'

Proposed Fix

In load_weights(path) in simple/ovl.py, check whether the specified file exists before attempting to load it:

if not Path(path).exists():
    sys.exit(f"weights {path} not found")

This would allow missing weights files to exit cleanly with an informative error message, matching how the rest of cmd_verify handles missing input files.

AI Usage Disclosure

• Identified with Gemini 3.8 Flash via Google Antigravity IDE.
• Reproduced locally and confirmed the FileNotFoundError behavior.