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/tests/encoder_tests.rs

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+use pixstrip_core::encoder::OutputEncoder;
+use pixstrip_core::types::{ImageFormat, QualityPreset};
+
+fn create_test_rgb_image(width: u32, height: u32) -> image::DynamicImage {
+    let img = image::RgbImage::from_fn(width, height, |x, y| {
+        image::Rgb([(x % 256) as u8, (y % 256) as u8, 128])
+    });
+    image::DynamicImage::ImageRgb8(img)
+}
+
+fn create_test_rgba_image(width: u32, height: u32) -> image::DynamicImage {
+    let img = image::RgbaImage::from_fn(width, height, |x, y| {
+        image::Rgba([(x % 256) as u8, (y % 256) as u8, 100, 255])
+    });
+    image::DynamicImage::ImageRgba8(img)
+}
+
+#[test]
+fn encode_jpeg() {
+    let img = create_test_rgb_image(200, 150);
+    let encoder = OutputEncoder::new();
+    let bytes = encoder.encode(&img, ImageFormat::Jpeg, Some(85)).unwrap();
+    assert!(!bytes.is_empty());
+    // JPEG magic bytes
+    assert_eq!(bytes[0], 0xFF);
+    assert_eq!(bytes[1], 0xD8);
+}
+
+#[test]
+fn encode_jpeg_quality_affects_size() {
+    let img = create_test_rgb_image(200, 150);
+    let encoder = OutputEncoder::new();
+    let high = encoder.encode(&img, ImageFormat::Jpeg, Some(95)).unwrap();
+    let low = encoder.encode(&img, ImageFormat::Jpeg, Some(50)).unwrap();
+    assert!(high.len() > low.len(), "Higher quality should produce larger files");
+}
+
+#[test]
+fn encode_png() {
+    let img = create_test_rgba_image(200, 150);
+    let encoder = OutputEncoder::new();
+    let bytes = encoder.encode(&img, ImageFormat::Png, None).unwrap();
+    assert!(!bytes.is_empty());
+    // PNG magic bytes
+    assert_eq!(&bytes[0..4], &[0x89, 0x50, 0x4E, 0x47]);
+}
+
+#[test]
+fn encode_webp() {
+    let img = create_test_rgb_image(200, 150);
+    let encoder = OutputEncoder::new();
+    let bytes = encoder.encode(&img, ImageFormat::WebP, Some(80)).unwrap();
+    assert!(!bytes.is_empty());
+    // RIFF header
+    assert_eq!(&bytes[0..4], b"RIFF");
+}
+
+#[test]
+fn encode_to_file() {
+    let img = create_test_rgb_image(200, 150);
+    let encoder = OutputEncoder::new();
+    let dir = tempfile::tempdir().unwrap();
+    let path = dir.path().join("output.jpg");
+    encoder.encode_to_file(&img, &path, ImageFormat::Jpeg, Some(85)).unwrap();
+    assert!(path.exists());
+    let metadata = std::fs::metadata(&path).unwrap();
+    assert!(metadata.len() > 0);
+}
+
+#[test]
+fn quality_from_preset() {
+    let encoder = OutputEncoder::new();
+    let preset = QualityPreset::High;
+    let q = encoder.quality_for_format(ImageFormat::Jpeg, &preset);
+    assert_eq!(q, 85);
+    let q = encoder.quality_for_format(ImageFormat::WebP, &preset);
+    assert!((q as f32 - 85.0).abs() < 1.0);
+}
+
+#[test]
+fn encode_gif_fallback() {
+    let img = create_test_rgba_image(50, 50);
+    let encoder = OutputEncoder::new();
+    let bytes = encoder.encode(&img, ImageFormat::Gif, None).unwrap();
+    assert!(!bytes.is_empty());
+    // GIF magic bytes
+    assert_eq!(&bytes[0..3], b"GIF");
+}

pixstrip-core/tests/integration_tests.rs

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+use pixstrip_core::encoder::OutputEncoder;
+use pixstrip_core::loader::ImageLoader;
+use pixstrip_core::operations::resize::resize_image;
+use pixstrip_core::operations::ResizeConfig;
+use pixstrip_core::types::ImageFormat;
+use std::path::Path;
+
+fn create_test_jpeg(path: &Path) {
+    let img = image::RgbImage::from_fn(400, 300, |x, y| {
+        image::Rgb([(x % 256) as u8, (y % 256) as u8, 128])
+    });
+    img.save_with_format(path, image::ImageFormat::Jpeg).unwrap();
+}
+
+fn create_test_png(path: &Path) {
+    let img = image::RgbaImage::from_fn(400, 300, |x, y| {
+        image::Rgba([(x % 256) as u8, (y % 256) as u8, 100, 255])
+    });
+    img.save_with_format(path, image::ImageFormat::Png).unwrap();
+}
+
+#[test]
+fn load_resize_save_as_webp() {
+    let dir = tempfile::tempdir().unwrap();
+    let input = dir.path().join("input.jpg");
+    let output = dir.path().join("output.webp");
+    create_test_jpeg(&input);
+
+    let loader = ImageLoader::new();
+    let img = loader.load_pixels(&input).unwrap();
+
+    let config = ResizeConfig::ByWidth(200);
+    let resized = resize_image(&img, &config).unwrap();
+    assert_eq!(resized.width(), 200);
+    assert_eq!(resized.height(), 150);
+
+    let encoder = OutputEncoder::new();
+    encoder.encode_to_file(&resized, &output, ImageFormat::WebP, Some(80)).unwrap();
+    assert!(output.exists());
+
+    let info = loader.load_info(&output).unwrap();
+    assert_eq!(info.dimensions.width, 200);
+    assert_eq!(info.dimensions.height, 150);
+}
+
+#[test]
+fn load_png_compress_smaller() {
+    let dir = tempfile::tempdir().unwrap();
+    let input = dir.path().join("input.png");
+    let output = dir.path().join("output.png");
+    create_test_png(&input);
+
+    let loader = ImageLoader::new();
+    let img = loader.load_pixels(&input).unwrap();
+
+    let encoder = OutputEncoder::new();
+    encoder.encode_to_file(&img, &output, ImageFormat::Png, None).unwrap();
+
+    let original_size = std::fs::metadata(&input).unwrap().len();
+    let optimized_size = std::fs::metadata(&output).unwrap().len();
+    assert!(
+        optimized_size <= original_size,
+        "Optimized PNG ({}) should be <= original ({})",
+        optimized_size,
+        original_size
+    );
+}
+
+#[test]
+fn load_jpeg_convert_to_png() {
+    let dir = tempfile::tempdir().unwrap();
+    let input = dir.path().join("input.jpg");
+    let output = dir.path().join("output.png");
+    create_test_jpeg(&input);
+
+    let loader = ImageLoader::new();
+    let img = loader.load_pixels(&input).unwrap();
+
+    let encoder = OutputEncoder::new();
+    encoder.encode_to_file(&img, &output, ImageFormat::Png, None).unwrap();
+    assert!(output.exists());
+
+    let info = loader.load_info(&output).unwrap();
+    assert_eq!(info.format, Some(ImageFormat::Png));
+}