Embed DQNConfig architecture hash (SHA-256 of state_dim, num_actions,
hidden_dims, dueling/distributional/noisy/IQN flags) in safetensors
file header on save. Validate hash on load to catch shape mismatches
before touching the VarMap - prevents silent corruption from loading
checkpoints trained with different network architectures.
All 5 save paths (trainable_adapter, trainer serialize_model,
DQNAgentType::save_checkpoint, RegimeConditionalDQN per-head) now
embed metadata. All 3 load paths (DQN::load_from_safetensors,
trainable_adapter::load_checkpoint, ensemble adapter) validate.
No backward compatibility: checkpoints without metadata are rejected
with a clear error message to re-train.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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