Add --hyperopt, --trials, and --parallel flags to `fxt train start` so
users can dispatch PSO hyperparameter optimization jobs alongside
regular training. The mode=hyperopt tag propagates through the service
layer which selects the correct binary (hyperopt_baseline_rl for DQN/PPO,
hyperopt_baseline_supervised for all others) and builds hyperopt-specific
CLI arguments via a new extra_args field on TrainingJobParams.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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