cleanup(dead): remove unused use_different_seeds field — [DEAD-001]

EnsembleConfig.use_different_seeds was set but never read anywhere in
the workspace. rg confirmed zero readers. Field removed from struct,
Default impl, and new() constructor (3 sites).

crates/ml-dqn/src/ensemble_network.rs

@@ -23,8 +23,6 @@ pub struct EnsembleConfig {
     pub base_config: QNetworkConfig,
     /// Number of networks in the ensemble
     pub num_networks: usize,
-    /// Whether to use different random seeds for each network
-    pub use_different_seeds: bool,
 }
 
 impl Default for EnsembleConfig {
@@ -32,7 +30,6 @@ impl Default for EnsembleConfig {
         Self {
             base_config: QNetworkConfig::default(),
             num_networks: 5,
-            use_different_seeds: true,
         }
     }
 }
@@ -88,7 +85,6 @@ impl EnsembleQNetwork {
         let ensemble_config = EnsembleConfig {
             base_config: config,
             num_networks,
-            use_different_seeds: true,
         };
 
         Ok(Self {