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,

server/src/atlas/step_canvas.rs

@@ -770,8 +770,8 @@ fn resolve_canvas_extent(
 /// every fixed rung (the canvas centre in world metres).
 ///
 /// **Row-space convention (T-1186 interaction — MUST READ):** the Global
-/// (rung-0) canvas uses **equator-anchored SIGNED region rows** (`wy = (row
-/// - rows/2) * REGION_M`, negative = north), matching the canonical
+/// (rung-0) canvas uses **equator-anchored SIGNED region rows**
+/// (`wy = (row - rows/2) * REGION_M`, negative = north), matching the canonical
 /// `derive_orbital_at_metres`/`derive_at_metres_with_riparian` convention
 /// (D-256(a): the ONE absolute-metre derive core; that core's own internal
 /// `district_pos` floor-division and `lat_frac` computation are both
@@ -1866,7 +1866,8 @@ mod tests {
             3,
             StepCanvasRung::Chunk.spacing_m(),
         );
-        let centre_i = 1 * 3 + 1;
+        let (centre_row, centre_col) = (1usize, 1usize);
+        let centre_i = centre_row * 3 + centre_col;
         assert_eq!(ids[centre_i], 3, "tie must resolve to the LOWEST city_id");
     }