Add SIMD-accelerated resize operation using fast_image_resize

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

pixstrip-core/src/operations/mod.rs

@@ -1,3 +1,5 @@
+pub mod resize;
+
 use serde::{Deserialize, Serialize};
 
 use crate::types::{Dimensions, ImageFormat, QualityPreset};

pixstrip-core/src/operations/resize.rs

@@ -0,0 +1,60 @@
+use fast_image_resize::{images::Image, Resizer, ResizeOptions, ResizeAlg, FilterType};
+
+use crate::error::{PixstripError, Result};
+use crate::types::Dimensions;
+
+use super::ResizeConfig;
+
+pub fn resize_image(
+    src: &image::DynamicImage,
+    config: &ResizeConfig,
+) -> Result<image::DynamicImage> {
+    let original = Dimensions {
+        width: src.width(),
+        height: src.height(),
+    };
+    let target = config.target_for(original);
+
+    if target.width == original.width && target.height == original.height {
+        return Ok(src.clone());
+    }
+
+    let src_rgba = src.to_rgba8();
+    let (src_w, src_h) = (src_rgba.width(), src_rgba.height());
+
+    let src_image = Image::from_vec_u8(
+        src_w,
+        src_h,
+        src_rgba.into_raw(),
+        fast_image_resize::PixelType::U8x4,
+    )
+    .map_err(|e| PixstripError::Processing {
+        operation: "resize".into(),
+        reason: format!("Failed to create source image: {}", e),
+    })?;
+
+    let mut dst_image = Image::new(
+        target.width,
+        target.height,
+        fast_image_resize::PixelType::U8x4,
+    );
+
+    let mut resizer = Resizer::new();
+    let options = ResizeOptions::new().resize_alg(ResizeAlg::Convolution(FilterType::Lanczos3));
+
+    resizer
+        .resize(&src_image, &mut dst_image, &options)
+        .map_err(|e| PixstripError::Processing {
+            operation: "resize".into(),
+            reason: format!("Resize failed: {}", e),
+        })?;
+
+    let result_buf: image::RgbaImage =
+        image::ImageBuffer::from_raw(target.width, target.height, dst_image.into_vec())
+            .ok_or_else(|| PixstripError::Processing {
+                operation: "resize".into(),
+                reason: "Failed to create output image buffer".into(),
+            })?;
+
+    Ok(image::DynamicImage::ImageRgba8(result_buf))
+}