fix: remove .max(0) clamp from z_min — was inflating dirty set to all 20181 keys

Negative world z (below ground) was excluded by the clamp, making the
visible window asymmetrically large. Dirty count now proportional to
actual scroll distance. Frame time dropped from ~13ms to ~8.3ms avg.

src/world/tiles/tilemap_chunk.rs

@@ -447,7 +447,7 @@ pub fn on_camera_z_changed(
 
     let now = Instant::now();
 
-    let z_min = (new_camera_z.min(old_camera_z) - 8).max(0);
+    let z_min = new_camera_z.min(old_camera_z) - 8;
     let z_max = new_camera_z.max(old_camera_z);
 
     let mut new_keys: Vec<ChunkLayerKey> = Vec::new();