refactor: sprite_mipmap_limit lives in sprites.rs

sprites.rs called back into gfx for this one function, one of the edges that
keep the client lib's modules in a single cycle. The function and its test
move to sprites.rs; gfx calls it there. No behaviour change.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

crates/horizon-client/src/gfx.rs

@@ -3197,7 +3197,7 @@ impl Renderer {
     /// The Mipmap Levels option: block textures sample at most `levels`
     /// reduced copies (0: none).
     pub fn set_mipmap_levels(&mut self, levels: u32) {
-        let levels = sprite_mipmap_limit(
+        let levels = crate::sprites::sprite_mipmap_limit(
             levels,
             self.sprite_textures[0].width(),
             self.sprite_minimum_size,
@@ -5479,11 +5479,6 @@ mod tests {
     }
 }
 
-pub(crate) fn sprite_mipmap_limit(requested: u32, size: u32, minimum: u32, available: u32) -> u32 {
-    // Only the atlas native minimum determines how many resampling-only mips to skip.
-    let upscale = (size / minimum.max(1)).max(1).ilog2();
-    requested.saturating_add(upscale).min(available - 1)
-}
 #[cfg(test)]
 mod sprite_upload_tests {
     use super::*;
@@ -5544,13 +5539,6 @@ mod sprite_upload_tests {
         }
     }
     #[test]
-    fn mipmap_option_keeps_real_16px_reductions() {
-        for levels in 0..=4 {
-            assert_eq!(sprite_mipmap_limit(levels, 32, 16, 6), levels + 1);
-            assert_eq!(sprite_mipmap_limit(levels, 16, 16, 5), levels);
-        }
-    }
-    #[test]
     fn prepared_texture_uploads_preserve_layer_order_and_settings() {
         let size = 4;
         let layers = 5;

crates/horizon-client/src/sprites.rs

@@ -342,10 +342,25 @@ pub(crate) fn test_frames(layer: u32, frames: Vec<Vec<u8>>, metadata: &str) -> S
     }
 }
 
+/// How many reduced copies a sprite atlas keeps: the Mipmap Levels option plus the
+/// resampling-only levels an upscaled pack needs, within what the atlas holds.
+pub(crate) fn sprite_mipmap_limit(requested: u32, size: u32, minimum: u32, available: u32) -> u32 {
+    // Only the atlas native minimum determines how many resampling-only mips to skip.
+    let upscale = (size / minimum.max(1)).max(1).ilog2();
+    requested.saturating_add(upscale).min(available - 1)
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
     #[test]
+    fn mipmap_option_keeps_real_16px_reductions() {
+        for levels in 0..=4 {
+            assert_eq!(sprite_mipmap_limit(levels, 32, 16, 6), levels + 1);
+            assert_eq!(sprite_mipmap_limit(levels, 16, 16, 5), levels);
+        }
+    }
+    #[test]
     fn identical_still_textures_share_a_layer() {
         let root =
             std::env::temp_dir().join(format!("horizon-sprite-share-{}", std::process::id()));
@@ -398,7 +413,7 @@ mod tests {
         assert_eq!((sprites.size, sprites.minimum_size), (32, 32));
         for level in 0..=4 {
             assert_eq!(
-                crate::gfx::sprite_mipmap_limit(level, sprites.size, sprites.minimum_size, 6),
+                sprite_mipmap_limit(level, sprites.size, sprites.minimum_size, 6),
                 level
             );
         }
@@ -408,7 +423,7 @@ mod tests {
         sprites.finish();
         assert_eq!((sprites.size, sprites.minimum_size), (32, 16));
         assert_eq!(
-            crate::gfx::sprite_mipmap_limit(0, sprites.size, sprites.minimum_size, 6),
+            sprite_mipmap_limit(0, sprites.size, sprites.minimum_size, 6),
             1
         );
     }

tools/bench_sprite_mips.py

@@ -52,7 +52,9 @@ def main():
     baseline = subprocess.check_output(['git', 'show', '89b58991bbc02eb79b6965fc4ca56357fe2297d9:crates/horizon-client/src/gfx.rs'], cwd=ROOT, text=True)
     code += '\n' + function(baseline, 'animate_sprites').replace('fn animate_sprites(', 'fn reference_animate_sprites(', 1)
     if args.test:
-        code += '\n' + function(source, 'sprite_mipmap_limit')
+        # sprites.rs holds it since the gfx -> sprites cut; older refs kept it in gfx.rs.
+        code += '\n' + (function(source, 'sprite_mipmap_limit') if 'fn sprite_mipmap_limit' in source
+                         else 'use sprites::sprite_mipmap_limit;')
         code += '\n' + source[source.index('#[cfg(test)]\nmod sprite_upload_tests'):]
     modules = '\n'.join(f'#[path = {json.dumps(str(ROOT / "crates/horizon-client/src" / (name + ".rs")))}] mod {name};'
                         for name in ('mipmap', 'sprites', 'fast_hash'))