Add SIMD-accelerated resize operation using fast_image_resize

Lanczos3 filter, supports ByWidth, ByHeight, FitInBox, Exact modes.
All 5 resize tests passing.

pixstrip-core/tests/resize_tests.rs

@@ -0,0 +1,61 @@
+use pixstrip_core::operations::resize::resize_image;
+use pixstrip_core::operations::ResizeConfig;
+use pixstrip_core::types::Dimensions;
+
+fn create_test_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)
+}
+
+#[test]
+fn resize_by_width() {
+    let img = create_test_image(4000, 3000);
+    let config = ResizeConfig::ByWidth(1200);
+    let result = resize_image(&img, &config).unwrap();
+    assert_eq!(result.width(), 1200);
+    assert_eq!(result.height(), 900);
+}
+
+#[test]
+fn resize_by_height() {
+    let img = create_test_image(4000, 3000);
+    let config = ResizeConfig::ByHeight(600);
+    let result = resize_image(&img, &config).unwrap();
+    assert_eq!(result.width(), 800);
+    assert_eq!(result.height(), 600);
+}
+
+#[test]
+fn resize_fit_in_box() {
+    let img = create_test_image(4000, 3000);
+    let config = ResizeConfig::FitInBox {
+        max: Dimensions { width: 1920, height: 1080 },
+        allow_upscale: false,
+    };
+    let result = resize_image(&img, &config).unwrap();
+    assert_eq!(result.width(), 1440);
+    assert_eq!(result.height(), 1080);
+}
+
+#[test]
+fn resize_no_upscale() {
+    let img = create_test_image(800, 600);
+    let config = ResizeConfig::FitInBox {
+        max: Dimensions { width: 1920, height: 1080 },
+        allow_upscale: false,
+    };
+    let result = resize_image(&img, &config).unwrap();
+    assert_eq!(result.width(), 800);
+    assert_eq!(result.height(), 600);
+}
+
+#[test]
+fn resize_exact() {
+    let img = create_test_image(4000, 3000);
+    let config = ResizeConfig::Exact(Dimensions { width: 1080, height: 1080 });
+    let result = resize_image(&img, &config).unwrap();
+    assert_eq!(result.width(), 1080);
+    assert_eq!(result.height(), 1080);
+}