style(simulation): gate bounce — box GenCompletion::BodyAnalyzed state; clippy/fmt sweeps

BodyWorldState grew past clippy's large-enum-variant threshold when it
began retaining Layer1Output (T-1170 Ruling 4b) — boxed, with the two
insert sites deref'd; manual_contains x2 (drainage tests),
cloned_ref_to_slice_refs x3 (river_course tests), fmt wraps.
gen_queue 18/18; clippy -D warnings clean; fmt --check clean.

Tickets: T-1170

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

server/src/atlas/plugin.rs

@@ -359,7 +359,7 @@ fn drain_generation_completions(
                         );
                     }
                 }
-                cache.insert(state);
+                cache.insert(*state);
             }
             GenCompletion::Failed { item, reason } => {
                 tracing::warn!(?item, %reason, "background generation work item failed");