Notebook: an all-RGB experiment should not need a channel label on "Your channels" #240

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

On "Your channels" (#233), a label must be declared in Step A even when every unit is RGB colour, where R/G/B are fixed and no channel label applies.

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

Fix

• When every unit is declared rgb, skip Step A, or mark it not applicable.
• The check-and-save screen then completes without any channel label.
• Check this against experiment-v1 §4.3 (RGB units) before changing anything.
• Test: an all-RGB fixture completes "Your channels" and screen 5 with no label declared.