Add output encoders (mozjpeg, oxipng, webp, avif) and integration tests

OutputEncoder dispatches to specialized encoders per format.
JPEG: mozjpeg with quality control. PNG: oxipng lossless optimization.
WebP: libwebp encoding. AVIF: ravif via image crate.
GIF/TIFF: fallback via image crate.
Phase 3 complete - 59 tests passing, zero clippy warnings.

pixstrip-core/src/lib.rs

@@ -1,5 +1,6 @@
 pub mod config;
 pub mod discovery;
+pub mod encoder;
 pub mod error;
 pub mod loader;
 pub mod operations;