route already means "which legs run"; a leg name is the most specific
route. Presets keep their meaning (all, the linux trio, windows-desktop,
macos-desktop, the bundled three); any other value selects legs by name,
so a leg name, a fragment of one, or the job name GitHub shows
("<leg> / e2e") runs just those legs. A route that selects nothing fails
instead of producing a green empty run.
The generator is now the one interpreter of route for source legs: the
linux/windows/macos jobs run when it selected legs for them, replacing
three hand-kept preset lists. Dispatch route becomes a string input (a
choice cannot take a leg name) and reaches the gen step through env, never
interpolated into the script.
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