Say what the drag payload actually is: the arming, not the cargo

`drag-payload` was documented as "called when a drag starts, so it
always reflects the selection as it is at that moment". Neither half is
true, and the comment is the only reason anyone would believe the drop
reads it.

`DragArea` tests `data.is_empty()` in its event filter — on every
pointer event, the first one included, which arrives long before there
is a drag. And a Slint binding that calls a callback has no dependency
to be invalidated on, so it is evaluated once, when a finger first lands
on that cell, and cached for the life of the cell. What it answers is
therefore always an empty selection.

None of the drop handlers read it; every one of them reads `dragging`,
which `drag-started` fills in at the moment that matters. What this
callback does is keep the `DragArea` armed, and it manages that only
because `set_user_data` is called unconditionally — an empty `Vec` is
still user data. Guarding that call, which reads as an obvious tidy-up,
would silently stop the grid dragging at all.

Comments only.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/gestures.md

@@ -89,7 +89,7 @@ This replaced a double tap, which had no visible state and could take forty phot
 
 "Any of them" is a union and "all of them" is an intersection. The tray is where both terms and the choice between them live, because a filter belongs on the filter bar.
 
-<sub>`ui/dr-ui/ui/library.slint:2093`</sub>
+<sub>`ui/dr-ui/ui/library.slint:2107`</sub>
 
 ### Resize the thumbnails
 
@@ -98,7 +98,7 @@ This replaced a double tap, which had no visible state and could take forty phot
 
 There is no wheel on a tablet, so without the pinch the cell size could only be changed by a control a finger cannot reach.
 
-<sub>`ui/dr-ui/ui/library.slint:2719`</sub>
+<sub>`ui/dr-ui/ui/library.slint:2733`</sub>
 
 ### File photographs in a collection
 
@@ -107,7 +107,7 @@ There is no wheel on a tablet, so without the pinch the cell size could only be
 
 The selection is what the drag carries, which is why selecting several is worth the mode: forty photographs file in one gesture.
 
-<sub>`ui/dr-ui/ui/library.slint:2878`</sub>
+<sub>`ui/dr-ui/ui/library.slint:2892`</sub>
 
 ### Open a photograph
 
@@ -116,7 +116,7 @@ The selection is what the drag carries, which is why selecting several is worth
 
 A tap opens; a tap that *moved* does not. Travel is what separates a deliberate tap from a hand brushing past, and it is the only thing that does: the two are the same length. An earlier version required the finger to dwell 120 ms instead, and that rejected ordinary taps — a real tap is often quicker than a brush.
 
-<sub>`ui/dr-ui/ui/library.slint:3145`</sub>
+<sub>`ui/dr-ui/ui/library.slint:3160`</sub>
 
 ### Rate a photograph without opening it
 
@@ -126,7 +126,7 @@ A tap opens; a tap that *moved* does not. Travel is what separates a deliberate
 
 A star has to take the press without it also reaching the cell, or every rating throws the user into develop.
 
-<sub>`ui/dr-ui/ui/library.slint:3257`</sub>
+<sub>`ui/dr-ui/ui/library.slint:3272`</sub>
 
 ### Drop the selection but keep selecting
 
@@ -135,7 +135,7 @@ A star has to take the press without it also reaching the cell, or every rating
 
 Distinct from Done, which leaves the mode entirely. Clearing keeps it, so the next selection can start straight away.
 
-<sub>`ui/dr-ui/ui/library.slint:3820`</sub>
+<sub>`ui/dr-ui/ui/library.slint:3835`</sub>
 
 ### Select everything the grid is showing
 
@@ -144,4 +144,4 @@ Distinct from Done, which leaves the mode entirely. Clearing keeps it, so the ne
 
 A scoped grid of two hundred frames is two hundred taps otherwise, and "all of them, except those three" is a far more common shape than the taps it took to say it.
 
-<sub>`ui/dr-ui/ui/library.slint:3837`</sub>
+<sub>`ui/dr-ui/ui/library.slint:3852`</sub>