Wire progressive JPEG, AVIF speed, and custom per-format quality to encoder

- Add EncoderOptions struct with progressive_jpeg and avif_speed fields
- Pass encoder options through ProcessingJob to PipelineExecutor
- mozjpeg set_progressive_mode() called when progressive JPEG enabled
- AVIF encoder speed now configurable (was hardcoded to 6)
- run_processing uses CompressConfig::Custom when user overrides preset defaults
- Executor properly handles AVIF quality and PNG level in Custom mode

pixstrip-core/src/preset.rs

@@ -40,6 +40,8 @@ impl Preset {
             watermark: self.watermark.clone(),
             rename: self.rename.clone(),
             preserve_directory_structure: false,
+            progressive_jpeg: false,
+            avif_speed: 6,
         }
     }