`habit.datasets`: demo pack and synthetic builders

Synthetic builders and the official downloadable demo imaging pack.

User guide: Python API guide (v2.0) · Habitat Guide Load from directory.

Official imaging pack (fetch_demo(), downloaded once) plus deterministic synthetic builders for tests and in-memory API exploration.

Classes

Filesystem inventory of a HABIT images/ + masks/ root.

`fetch_demo`	Download the official preprocessed demo pack once and return its root.
`get_data_home`	Return the cache directory for downloaded HABIT demo packs.
`inspect_preprocessed_root`	List subjects and series folders under a HABIT preprocessed root.
`make_synthetic_cohort`	Build a deterministic imaging cohort entirely in memory.
`make_synthetic_feature_table`	Build a deterministic tabular dataset for fast ML golden tests.