style: apply rustfmt across all crates and demos

Consistent formatting pass: line wrapping, import sorting, trailing
whitespace removal, let-chain indentation, merged derive attributes,
and unsafe block reformatting.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

demos/rtx-cellatlas-demo/src/clustering.rs

@@ -485,7 +485,10 @@ fn get_cluster_markers(cluster_id: usize) -> Vec<String> {
     ];
 
     let idx = cluster_id % all_markers.len();
-    all_markers[idx].iter().map(std::string::ToString::to_string).collect()
+    all_markers[idx]
+        .iter()
+        .map(std::string::ToString::to_string)
+        .collect()
 }
 
 #[cfg(test)]