ml_make_seq_model: Deterministic sequence model factory (GRU/LSTM/CNN1D with linear fallback)
Description
Returns fit/predict closures for sequence models that consume flattened
tabular inputs (n \(\times\) (steps \(\times\) p)) and internally reshape to (n, steps, p).
If Keras/TensorFlow is unavailable, falls back to a linear baseline so
examples remain runnable on CPU-only machines.