Fix remaining clippy warnings in ringkernel-core

- Use checked_div() instead of manual division check in multi_gpu.rs
- Use sort_by_key() with Reverse instead of sort_by() in observability.rs

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: mivertowski

crates/ringkernel-core/src/multi_gpu.rs

@@ -1685,16 +1685,14 @@ impl KernelMigrator {
             successful_migrations: successful,
             failed_migrations: failed,
             bytes_transferred: self.stats.bytes_transferred.load(Ordering::Relaxed),
-            avg_checkpoint_time: if total > 0 {
-                Duration::from_micros(checkpoint_us / total)
-            } else {
-                Duration::ZERO
-            },
-            avg_restore_time: if total > 0 {
-                Duration::from_micros(restore_us / total)
-            } else {
-                Duration::ZERO
-            },
+            avg_checkpoint_time: checkpoint_us
+                .checked_div(total)
+                .map(Duration::from_micros)
+                .unwrap_or(Duration::ZERO),
+            avg_restore_time: restore_us
+                .checked_div(total)
+                .map(Duration::from_micros)
+                .unwrap_or(Duration::ZERO),
         }
     }
 }

crates/ringkernel-core/src/observability.rs

@@ -2129,7 +2129,7 @@ impl GpuMemoryDashboard {
         // Top allocations by size
         let allocations = self.allocations.read();
         let mut sorted_allocs: Vec<_> = allocations.values().collect();
-        sorted_allocs.sort_by(|a, b| b.size.cmp(&a.size));
+        sorted_allocs.sort_by_key(|a| std::cmp::Reverse(a.size));
 
         if !sorted_allocs.is_empty() {
             writeln!(report).unwrap();