Add graceful degradation and scan API for robust scanning

[dguido]

Summary

• Adds ScanResult class with is_safe, severity, results, and errors attributes
• Adds scan_file() function for graceful single-file scanning
• Adds scan_archive() function for scanning ZIP archives with graceful error handling
• Adds RelaxedZipFile class that ignores CRC validation errors (matches PyTorch behavior)
• Exports new functions from package for easy programmatic use

API Example

import fickling

# Scan a single file
result = fickling.scan_file("model.pkl")
if not result:
    print(f"Unsafe: {result.severity}")
    for error in result.errors:
        print(f"  Error: {error}")

# Scan an archive
results = fickling.scan_archive("models.zip")
for name, result in results.items():
    print(f"{name}: {'safe' if result else 'unsafe'}")

Test plan

• All existing tests pass
• Linters pass
• Manual testing with corrupted archives

🤖 Generated with Claude Code