fix: kNN zero-variance dims get inf std; hot-start SAC from saved model

- nucon/model.py: constant input dimensions (zero variance in training
  data) now get std=inf so they contribute 0 to normalised kNN distance
  instead of causing catastrophic OOD from tiny float epsilon
- scripts/train_sac.py: add --load, --steps, --out CLI args; --load
  hot-starts actor/critic weights from a previous run (learning_starts=0)

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

nucon/model.py

@@ -95,7 +95,10 @@ class ReactorKNNModel:
         self._raw_states = np.array(raw)
         self._rates      = np.array(rates)
         self._mean = self._raw_states.mean(axis=0)
-        self._std  = self._raw_states.std(axis=0) + 1e-8
+        raw_std = self._raw_states.std(axis=0)
+        # Dimensions with zero variance in the training data carry no distance information.
+        # Use inf so they contribute 0 to normalised L2 (i.e., are ignored in kNN lookup).
+        self._std  = np.where(raw_std < 1e-6, np.inf, raw_std)
         self._states = (self._raw_states - self._mean) / self._std
 
     def _lookup(self, s: np.ndarray):

scripts/train_sac.py

@@ -2,12 +2,14 @@
 
 Usage:
     python3.14 train_sac.py
+    python3.14 train_sac.py --load /tmp/sac_nucon_knn  # hot-start from previous run
 
 Requirements:
     - NuCon game running (for parameter metadata)
     - /tmp/reactor_knn.pkl  (kNN-GP model)
     - /tmp/nucon_dataset.pkl  (500-sample dataset for init_states)
 """
+import argparse
 import pickle
 from gymnasium.wrappers import TimeLimit
 from stable_baselines3 import SAC
@@ -16,6 +18,12 @@ from stable_baselines3.her.her_replay_buffer import HerReplayBuffer
 from nucon.sim import NuconSimulator
 from nucon.rl import NuconGoalEnv, Parameterized_Objectives, Parameterized_Terminators
 
+parser = argparse.ArgumentParser()
+parser.add_argument('--load', default=None, help='Path to existing model to hot-start from')
+parser.add_argument('--steps', type=int, default=50_000, help='Total timesteps (default: 50000)')
+parser.add_argument('--out', default='/tmp/sac_nucon_knn', help='Output path for saved model')
+args = parser.parse_args()
+
 # ---------------------------------------------------------------------------
 # Load model and dataset
 # ---------------------------------------------------------------------------
@@ -59,6 +67,14 @@ env = TimeLimit(env, max_episode_steps=MAX_EPISODE_STEPS)
 # before the first gradient step. As the policy learns to stay in-dist, episodes
 # will get longer and HER has more transitions to relabel.
 # ---------------------------------------------------------------------------
+if args.load:
+    print(f"Hot-starting from {args.load}")
+    model = SAC.load(args.load, env=env, device='auto',
+                     custom_objects={'learning_rate': 3e-4, 'batch_size': BATCH_SIZE,
+                                     'tau': 0.005, 'gamma': 0.98,
+                                     'train_freq': 64, 'gradient_steps': 8,
+                                     'learning_starts': 0})
+else:
     model = SAC(
         'MultiInputPolicy',
         env,
@@ -78,6 +94,6 @@ model = SAC(
         device='auto',
     )
 
-model.learn(total_timesteps=50_000)
+model.learn(total_timesteps=args.steps)
-model.save('/tmp/sac_nucon_knn')
+model.save(args.out)
-print("Saved to /tmp/sac_nucon_knn.zip")
+print(f"Saved to {args.out}.zip")