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/gestures.md

@@ -111,7 +111,7 @@ Noise reduction and capture sharpening are judgements about single pixels, and a
 
 The edit on screen is saved on the way out, so stepping through a folder is as much a departure as going back to the grid and loses nothing.
 
-<sub>`ui/dr-ui/ui/app.slint:2500`</sub>
+<sub>`ui/dr-ui/ui/app.slint:2522`</sub>
 
 ### See the photograph before you edited it
 
@@ -121,12 +121,13 @@ The edit on screen is saved on the way out, so stepping through a folder is as m
 
 Held rather than toggled, and no split screen: a split halves the working image on the tablet the column was sized for, and the comparison photographers describe making is a flick back and forth. It takes no history step, so checking whether a frame is overcooked costs nothing to undo afterwards.
 
-<sub>`ui/dr-ui/ui/app.slint:2624`</sub>
+<sub>`ui/dr-ui/ui/app.slint:2646`</sub>
 
 ### Put one control back to its default
 
 - **Touch** — Double-tap its track
 - **Pointer** — Double-click its track, or right-click it
+- **Keyboard** — R, for the control last moved
 
 The column is 280px wide and the colour mixer alone puts thirty-six of these in it, so a reset button per row would be most of the width. Two ways in with a pointer because right-click is the one a hand already reaches for and double-click is the one that needs no second button. A group's own reset is in its heading; this is the single control.
 
@@ -136,8 +137,9 @@ The column is 280px wide and the colour mixer alone puts thirty-six of these in
 
 - **Touch** — Tap the ring at the head of its row
 - **Pointer** — Click the ring at the head of its row
+- **Keyboard** — H, for the selected layer history step, unlike holding "Before" — the layer really is off until it is switched back on.
 
-Disabling a layer is the before-and-after a local edit constantly wants, so it is one press away rather than inside the row. It is an edit and does take a history step, unlike holding "Before" — the layer really is off until it is switched back on.
+Disabling a layer is the before-and-after a local edit constantly wants, so it is one press away rather than inside the row. It is an edit and does take a
 
 <sub>`ui/dr-ui/ui/masks.slint:209`</sub>
 
@@ -148,7 +150,7 @@ Disabling a layer is the before-and-after a local edit constantly wants, so it i
 
 A mask is judged by seeing where it falls, and two are judged by seeing where they meet — so each row has its own eye rather than the panel having one, and the eye is drawn in the colour the mask shows in, so the row says which shape on the picture is its. Nothing about the edit changes: this is how the photograph is looked at, and takes no history step.
 
-<sub>`ui/dr-ui/ui/masks.slint:274`</sub>
+<sub>`ui/dr-ui/ui/masks.slint:275`</sub>
 
 ## Collections sidebar
 

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