CheXpert loader with device labels

[sebasmos]

Load CheXpert into the shared schema and expose its Support Devices label for the later natural-cue validation arm.

Reuse vs bespoke. Reuse the standard data library.

Definition of done. A sample loads with device labels available for stratification.

Part of: #4

[sebasmos]

Initial code landed in 1ba8288: loader + registry landed. Suite: 189 passed, 5 skipped (optional-dep paths), ruff clean.

Keeping open: CheXpert raw->Case adapter (with Support Devices) is a stub for the owner.

[sebasmos]

Parser implemented in : CheXpert adapter implemented incl. Support Devices flag (fixture-tested). 292 passed, ruff clean (PR #66).

Keeping open: verify on the real data (point raw_root at the credentialed/large dataset and confirm the row-level parse), which is the assigned owner's step.