Described runs with several layouts: inconsistent_mapping and the missing suggested mapping hide the real refusal #245

Found while reviewing #243 (follow-up of #239).

A described experiment with an RGB unit and a fluorescence unit (Spec fixture v29) now passes the notebook folder check and stops with R_HETEROGENEOUS_LAYOUT_NOT_ENABLED, as the Spec requires. Two messages around it still point the user the wrong way:

• run_experiment with the mapping CYX reports inconsistent_mapping first ("The axis mapping (CYX) does not fit 3 images: their dimensions are arranged differently", with the advice to export the images the same way), then R_HETEROGENEOUS_LAYOUT_NOT_ENABLED. Re-exporting would not help: the RGB unit is a different layout, and this version runs one layout per experiment.
• preflight(...).suggested_mapping is None for this experiment, so the notebook's axis step offers no roles. It does not say that this comes from the two layouts.

Reproduce: the mixed_experiment fixture of tests/test_described_unit_checks.py with v29 published, then run_experiment(root, prefill_mapping("CYX"), output_folder=out).

Possible fix (needs an owner decision): when the description is heterogeneous, put R_HETEROGENEOUS_LAYOUT_NOT_ENABLED first. Then either leave out or annotate the axis-review issues of images that belong to a layout other than the one the mapping describes. The axis step could also say that no roles are suggested because the units declare different layouts. Nothing that is refused today may be accepted.