The Edit menu gains Toggle Exclude, next to Add/Delete scene. An excluded
scene stays on the timeline, dimmed in the strip, and Generate All skips it
-- both in its count and its loop, so "2 of 2 did not generate" never counts
a scene that was never going to run.
Covered by a jsdom render test, a real end-to-end menu-to-strip test, and a
Generate All exclude test.