Apply output DPI to JPEG files via mozjpeg pixel density

- Add output_dpi field to EncoderOptions
- Set JFIF pixel density header in JPEG output when DPI > 0
- Wire output_dpi from ProcessingJob through to encoder in both
  parallel and sequential execution paths

pixstrip-core/src/encoder.rs

@@ -8,6 +8,8 @@ use crate::types::{ImageFormat, QualityPreset};
 pub struct EncoderOptions {
     pub progressive_jpeg: bool,
     pub avif_speed: u8,
+    /// Output DPI (0 means don't set / use default)
+    pub output_dpi: u32,
 }
 
 pub struct OutputEncoder {
@@ -20,6 +22,7 @@ impl OutputEncoder {
             options: EncoderOptions {
                 progressive_jpeg: false,
                 avif_speed: 6,
+                output_dpi: 0,
             },
         }
     }
@@ -80,6 +83,13 @@ impl OutputEncoder {
         if self.options.progressive_jpeg {
             comp.set_progressive_mode();
         }
+        if self.options.output_dpi > 0 {
+            comp.set_pixel_density(mozjpeg::PixelDensity {
+                unit: mozjpeg::PixelDensityUnit::Inches,
+                x: self.options.output_dpi as u16,
+                y: self.options.output_dpi as u16,
+            });
+        }
 
         let mut output = Vec::new();
         let mut started = comp.start_compress(&mut output).map_err(|e| PixstripError::Processing {