- Return ErrUnsupportedOutputFormat for WebP/AVIF encoding
- Return ErrInvalidFitMode for unknown fit mode values
- Add ValidateFitMode() for input validation
- Validate fit mode at handler level before processing
Silent fallbacks violate the principle of least surprise and mask bugs.
When a user explicitly specifies a value, we should either honor it or
return an error - never silently substitute a different value.
- StoreOutput now returns output hash for immediate retrieval
- Cache misses now serve from disk file after storing (same as hits)
- Log served_bytes from actual io.Copy result (avoids stat calls)
- Remove ContentLength field usage for cache hits (stream from file)
- Fix tests to properly check all return values
