The GPU experience collector gate checked only `dqn.dueling_q_network`
(plain dueling), but with both `use_dueling: true` AND `use_distributional: true`
(the defaults), DQN creates hybrid `dist_dueling_q_network` instead, leaving
the plain dueling fields as None. This meant the GPU collector never initialized
despite curiosity being enabled.
Fix: Add else-if fallback to check `dist_dueling_q_network`/`dist_dueling_target_network`
when plain dueling fields are None. Same fix applied to the weight sync site.
Also fix test_train_with_empty_data_completes_gracefully: reduce to 5 epochs with
early stopping disabled. The debug-mode async state machine is large enough that
empty-data epochs run ~180ms each (vs ~3ms in release), triggering both plateau
and patience-based early stopping. The test purpose is crash-freedom, not timing.
2497 tests pass, 0 clippy warnings.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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