Bind the two develop gestures that were described and not bound

FR-DEV-16's book said resetting a control and hiding a mask layer were
reachable by pointer and by finger, and stopped there. The reason was
honest: the generated rows have no focus, so "reset the focused control"
named a thing the panel could not point at. But a photographer at the
keyboard means something narrower than focus. They mean the slider they
just dragged too far, and that is a thing the panel can remember.

So the Adjustments global keeps the last control moved — two indices,
written where the panel forwards the change and cleared when the next
photograph opens, so a reset cannot reach back into the previous edit
through an index that happens to be shared. R puts it back, through the
same callback the track's double-click takes, and is silent until
something has moved.

The mask layer needs no such notion, because the panel already has a
selection: the rows the edge controls point at. H hides or shows those,
through the path the ring at the head of the row takes, so it is an edit
and a history step exactly as the ring is. A mixed selection goes to
shown, since the layer nobody can see is the one being asked about.

Both tags now carry the key, and the book says so.

docs/requirements.md

@@ -533,6 +533,17 @@ which knows nothing of a layer's shaping and nothing at all about a gradient, a
 so choosing a subject or a category produced a layer whose extent was invisible, and every control
 in FR-DEV-19a and FR-DEV-19b acted on something the photographer could not see.
 
+**FR-DEV-16 — A keyboard vocabulary for develop.** Every develop gesture that can be reached from
+the keyboard is bound, tagged beside its implementation, and generated into the gesture book
+(FR-UI-4): stepping through the folder, fit and 1:1, holding the original, undo and redo, copying
+and pasting settings, cycling the adjustment groups, resetting the control last moved, and showing
+or hiding the selected mask layer. None is keyboard-only — each has a pointer and a touch route
+(FR-DEV-3b) — and the book is regenerated from the tags, so it cannot describe a binding the
+application does not have.
+Editing rhythm depends on the hands staying put: reaching for a menu breaks the concentration of an
+edit the same way it breaks the pace of a cull. A binding nobody can discover is the same as no
+binding, which is why the generated book is part of the requirement rather than documentation of it.
+
 ### 3.4 Display and interaction
 
 **FR-DSP-1 — Proxy-resolution rendering.** The develop view renders at the resolution actually