Described runs: allow RGB and fluorescence units in one experiment #239

Found while building notebook PR 16 (#237/#238).

The experiment description (experiment-v1) declares the image type per unit, so a described experiment may mix RGB colour units with fluorescence (channels) units. The folder check refuses such a mix before the notebook screens are reached, with mixed_dtypes and inconsistent_dimensions. The new screens therefore can't be used on that experiment, and the mixed rows of the channel grid are covered only by library tests.

Owner decision (2026-11-09): file this and fix it after PR 16.

Fix

• In a described run, apply the dtype and dimension consistency checks per unit, according to its declared image_type, instead of across the whole experiment.
• Undescribed (v0.4) runs keep today's refusal.
• Test: a mixed RGB and fluorescence fixture reaches the channel grid, screen 5 and the run. The refusal on undescribed runs is unchanged.