style(simulation): clippy nits on the step-canvas batch (gate bounce)

Two needless mutable borrows in the serve system, one spurious
markdown list marker from a doc-comment line wrap (a leading '- '
turned the whole T-1186 convention note into a lazy continuation), and
a named row/col pair replacing the identity-op grid index in the
settlement-id tie test.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

server/src/atlas/plugin.rs

@@ -268,8 +268,8 @@ fn serve_step_canvas_requests(
         let resp = match resolver.as_ref() {
             Some(r) => serve_step_canvas_request(
                 &req,
-                &mut global_cache.as_mut(),
-                &mut canvas_cache.as_mut(),
+                global_cache.as_mut(),
+                canvas_cache.as_mut(),
                 &queue,
                 &r.0,
                 params_reader,