feat: add min_hold_bars config (default 5) — prevents per-bar churning

Add min_hold_bars to DQNHyperparameters (usize, default 5), ExperienceSection
in training_profile, and both TOML configs (smoketest=3, production=5).
Wired through apply_to() so TOML overrides land correctly.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

config/training/dqn-production.toml

@@ -68,6 +68,7 @@ min_loss_improvement_pct = 0.1
 [experience]
 initial_capital = 100000.0
 tx_cost_multiplier = 0.0001
+min_hold_bars = 5
 
 [experience.fill_simulation]
 ioc_fill_prob = 0.85

config/training/dqn-smoketest.toml

@@ -37,6 +37,7 @@ min_epochs_before_stopping = 5
 [experience]
 gpu_n_episodes = 32
 gpu_timesteps_per_episode = 100
+min_hold_bars = 3
 
 [experience.fill_simulation]
 ioc_fill_prob = 0.85