training/fleet/manifest: accept knn + knn_semi in _ALLOWED_MODELS

Validator's allowed-models frozenset was missing knn and knn_semi
even though the manifest gained those jobs and the model registry
registered the classes. Lambda bootstrap blocked at:
  TrainingManifestError: job 'knn-realistic': model 'knn' not in
    ['cnn', 'gbt', 'gru', 'lstm', 'mlp', 'transformer', 'transformer_ssl']

Now {gbt, knn, knn_semi, mlp, cnn, gru, lstm, transformer, transformer_ssl}.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

training/fleet/manifest.py

@@ -110,7 +110,8 @@ class TrainingManifest:
 
 # Allowed model names — keep in sync with training/models/REGISTRY
 _ALLOWED_MODELS = frozenset({
-    "gbt", "mlp", "cnn", "gru", "lstm", "transformer", "transformer_ssl",
+    "gbt", "knn", "knn_semi",
+    "mlp", "cnn", "gru", "lstm", "transformer", "transformer_ssl",
 })
 _ALLOWED_MODES = frozenset({"realistic", "oracle"})
 _ALLOWED_RECIPES = frozenset({"host", "sample", "time"})