fix(ml-dqn): make is_gpu_prioritized() available without cuda feature

Move the cfg gate inside the method body instead of gating the entire
function. Without cuda, the GpuPrioritized variant doesn't exist so
matches! returns false — no need for separate cfg blocks at call sites.

Fixes compile-services CI failure (services build without --features cuda).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

crates/ml-dqn/src/replay_buffer_type.rs

@@ -490,10 +490,13 @@ impl ReplayBufferType {
         matches!(self, Self::Prioritized(_))
     }
 
-    /// Check if using GPU-resident prioritized replay
-    #[cfg(feature = "cuda")]
+    /// Check if using GPU-resident prioritized replay.
+    /// Returns false when compiled without `cuda` feature (variant doesn't exist).
     pub const fn is_gpu_prioritized(&self) -> bool {
-        matches!(self, Self::GpuPrioritized(_))
+        #[cfg(feature = "cuda")]
+        { matches!(self, Self::GpuPrioritized(_)) }
+        #[cfg(not(feature = "cuda"))]
+        { false }
     }
 
     /// Adaptive buffer sizing based on epsilon decay