specify width when laying out 3 containers wide

[#55426982]

The layout was failing for the embedded and full-screen
latent heat Interactives when the width of the third
component was not specified directly.

Also add example interactives in ./src/interactives/layout-tests
use-once pattern: latent-heat "Start Heating" buttons
latent-heat cooling: fix use-once button references
allow digits to be used in template container ids
semantic layout: additional comments
model divergence with extra-long DNA

[#55660284]

Open this Interactive and click "Translate" button.
TODO note: model shouldn't issue messages directly
fix typo (labQuest -> LabQuest)
Allow sensorType to be null

IS [#55583664]
"Select type of sensor…" option.
sensor interactive: don't set default sensor

IS [#55583664]