The help sheet says which move does a thing, and the manual has a
picture of the thing being done, but nothing joined the two: a user
reading "Pinch it with two fingers" had no way from there to the GIF of
it.
A GESTURE tag takes an optional `manual:` field naming a heading of
docs/manual/README.md by its anchor. The scan checks every one against
the anchors the bundled page is rendered with and fails when the manual
has no such heading, so renaming a section cannot leave the sheet
linking to the top of the page; gestures-check carries the same failure
into CI. The anchor goes into gesture_book.rs as a new field, and into
docs/gestures.md as a "See it" link to manual/README.md#anchor. The help
sheet draws a "See it" button beside the title of each gesture that has
one, which opens the bundled manual at that section.
The field is additive: a tag without it is unchanged, and no gesture
carries one yet.
dtourolle
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