X13: Adds PerceptionTrainer::save_checkpoint as a thin delegate to
self.trunk.save_checkpoint. Inference-only serialization — grads + AdamW
state aren't included.
X14: Inside the existing auc_h6000_improved block in alpha_train.rs,
calls trainer.save_checkpoint(out_dir / 'trunk_best_h6000.bin') so the
trained trunk lands alongside alpha_train_summary.json. Extends
AlphaTrainSummary with best_h6000_ckpt_path (Option<String>) so
downstream tooling (fxt-backtest --checkpoint) can locate the file
without re-deriving the path.
After this commit, every alpha-perception Argo workflow run produces
a CheckpointV2 file at every new-best-h6000 epoch, ready for backtest
consumption.
Verification:
- ml-alpha lib tests: 34 pass
- alpha_train example builds clean (release)
Per spec §1.1 (X13+X14).
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