from_checkpoint was creating fresh models with random weights and setting
is_trained=true, allowing trading on noise. Now validates checkpoint exists
and does NOT set is_trained=true when weights are random.
Mamba2 predict errors are now wrapped as MLError::InferenceError so the
fallback manager can degrade model health. Both TFT and Mamba2 predict
refuse to run on untrained models, and is_ready() reflects trained state.
Also fixes TFT input_dim mismatch (16 vs 5+10+16=31).
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>