M17: ImageJ/FIJI feature parity batch 2 — 8 more features (327 tests)

1. File Formats: FITS reader (BITPIX 8/16/32/-32/-64, BZERO/BSCALE),
   AVI reader/writer (uncompressed RIFF), raw binary import
2. Image Calculator: 13 operations (Add/Sub/Mul/Div/AND/OR/XOR/Min/Max/
   Average/Difference/Copy/Transparent-zero), stack calculator
3. Auto-Threshold: 12 new methods (Huang, Intermodes, Li, MaxEntropy,
   Mean, MinError, Minimum, Moments, Percentile, RenyiEntropy, Shanbhag, Yen)
4. Background: sliding paraboloid, create_background, light_background option
5. Multi-point Counter: cell counting with categories, CSV export, auto-detect
6. Stack Statistics: per-slice stats, z/t profiles, kymograph extraction
7. Annotations (new ri-annotation crate): text/arrow/scale_bar with 8x8
   bitmap font, burn_annotations to pixel data
8. Batch Processing: process_directory, convert_directory, batch_measure,
   recursive processing with progress callbacks

crates/ri-io/src/lib.rs

@@ -1,5 +1,8 @@
+pub mod avi;
 pub mod dicom;
+pub mod fits;
 pub mod nifti;
+pub mod raw_import;
 
 use std::path::Path;
 
@@ -87,6 +90,9 @@ pub fn save_image(
     }
 
     match ext.as_str() {
+        "avi" => {
+            return avi::save_avi(hyperstack, path);
+        }
         "png" => {
             let file = std::fs::File::create(path)?;
             let mut encoder = png::Encoder::new(file, w, h);
@@ -148,6 +154,12 @@ pub fn load_image(path: &Path) -> Result<Hyperstack, RiError> {
     if path_lower.ends_with(".nii") || path_lower.ends_with(".nii.gz") {
         return nifti::load_nifti(path);
     }
+    if path_lower.ends_with(".fits") || path_lower.ends_with(".fit") || path_lower.ends_with(".fts") {
+        return fits::load_fits(path);
+    }
+    if path_lower.ends_with(".avi") {
+        return avi::load_avi(path);
+    }
 
     let img = image::open(path).map_err(|e| RiError::ImageDecode(e.to_string()))?;
     let file_size_bytes = std::fs::metadata(path).map(|m| m.len()).ok();