cleanup(fflag): delete dead use_spectral_norm in RainbowNetworkConfig — [FFLAG-002]

RainbowNetworkConfig.use_spectral_norm + spectral_norm_iterations were
declared but never read anywhere in the workspace. Defaulted false, set
false at the sole construction site. Pure write-only pair. Deleted both
fields + 2 construction sites. Serde default (no deny_unknown_fields)
keeps old JSON configs deserializable.

crates/ml-dqn/src/rainbow_config.rs

@@ -124,8 +124,6 @@ impl Default for RainbowDQNConfig {
                 distributional: DistributionalConfig::default(),
                 noisy_sigma_init: 0.5,
                 dueling: true,
-                use_spectral_norm: false,
-                spectral_norm_iterations: 1,
             },
             distributional: DistributionalConfig {
                 num_atoms: 51,

crates/ml-dqn/src/rainbow_network.rs

@@ -39,8 +39,6 @@ pub struct RainbowNetworkConfig {
     /// Initial noise std dev for noisy layers (Rainbow DQN default: 0.5, scaled by 1/sqrt(in))
     pub noisy_sigma_init: f64,
     pub dueling: bool,
-    pub use_spectral_norm: bool, // Q-value stability feature
-    pub spectral_norm_iterations: usize,
 }
 
 impl Default for RainbowNetworkConfig {
@@ -54,8 +52,6 @@ impl Default for RainbowNetworkConfig {
             distributional: DistributionalConfig::default(),
             noisy_sigma_init: 0.5,
             dueling: true,
-            use_spectral_norm: false, // Disabled by default, enable for unstable training
-            spectral_norm_iterations: 1,
         }
     }
 }