Wire the develop panels through globals, so a second copy is one line

Every panel in the develop column declared its inputs and its callbacks and
had `app.slint` bind each one to a property or a callback on the window root.
That is fine while a panel is drawn once. N9 draws them a second time, in the
portrait dock, and the wiring is what would have to be copied: `MaskPanel`
alone ran to forty lines of forwarding, and a callback added to one copy and
not the other compiles, renders, and simply does nothing on the layout nobody
was looking at.

So the wiring moved to Slint globals. A panel reads the global and calls the
global; Rust hooks the global instead of the window; and the instantiation in
the column is now the panel's name and a pair of braces — every one of the ten
children of the column, with no property that differs by placement left to
supply.

There is a global per panel family rather than one for all of them, and the
reason is an import cycle. Each panel's model struct — `ParamRow`, `MaskRow`,
`HistogramView` — is declared in the panel's own file, so a single global
holding `[MaskRow]` and `[ParamRow]` would have to live in a file importing
`masks.slint` and `adjust.slint` while both imported the global back, which
Slint rejects. Breaking that needs six model declarations relocated, which is a
change to the data model and not to the plumbing this is about. A global beside
the panel it serves also lets each name drop the prefix it was carrying only
because the window root is one flat namespace: `root.spot-radius` is
`Repair.radius`, and `root.peaking-on` is `Peaking.showing`.

`session.slint` is new and holds the two facts every family needs and none of
them owns: whether there is an open photograph to edit, and which mode the view
is in, with the three readings of the mode derived once instead of at each of
the dozen places that tested one. `ViewMode` moves there from `adjust.slint`,
where it was only ever a lodger.

Nothing on screen changes. What is not here: the tool rail and the status strip
still take their properties at the instantiation, because they are drawn once
and N9 does not copy them; the preset sheet's own state stays on the window,
because the library grid opens the same sheet and a global cannot bind the
window's state — which is why `Transfer.open-presets` is handled in
`presets.rs`, beside the summary it already had to compute.

docs/gestures.md

@@ -5,7 +5,7 @@
 
 Every entry here is extracted from the comment beside the code that implements it, so this file cannot describe a gesture the application does not have. Add one by writing a `GESTURE:` block next to the implementation; there is nowhere else to write it.
 
-31 gestures, in 3 places.
+30 gestures, in 3 places.
 
 ## Develop
 
@@ -16,7 +16,7 @@ Every entry here is extracted from the comment beside the code that implements i
 
 Sampling a neutral is the first move of the tonal pass — every colour judgement afterwards is measured against where the grey was put — and guessing at two sliders until a wall stops looking green is the wrong way round. One click is one sample and one step to undo; the sliders stay, because a sampled neutral is where the decision starts rather than where it ends.
 
-<sub>`ui/dr-ui/ui/adjust.slint:157`</sub>
+<sub>`ui/dr-ui/ui/adjust.slint:158`</sub>
 
 ### Magnify the photograph by any amount
 
@@ -25,16 +25,7 @@ Sampling a neutral is the first move of the tonal pass — every colour judgemen
 
 Anchored on the fingers' midpoint, and on the pointer, so the gesture reads as magnifying the picture rather than sliding it about. Double-tap is the way to an exact 1:1; this is the way to everything in between.
 
-<sub>`ui/dr-ui/ui/app.slint:2018`</sub>
-
-### Paint a mask by hand
-
-- **Touch** — Choose Paint or Erase, then drag on the photograph
-- **Pointer** — Choose Paint or Erase, then drag
-
-A model's mask stops inside a shoulder and leaks into the hair, and no single edge control fixes two errors that go opposite ways. The whole stroke is one step in the history, so taking a mark back costs one press however long it took to make.
-
-<sub>`ui/dr-ui/ui/app.slint:2091`</sub>
+<sub>`ui/dr-ui/ui/app.slint:1849`</sub>
 
 ### Move a magnified photograph about
 
@@ -43,7 +34,7 @@ A model's mask stops inside a shoulder and leaks into the hair, and no single ed
 
 Only once there is something outside the viewport to reach, which is why the cursor becomes a hand exactly then. The view is clamped to the frame: panning past the edge would show undefined area beside the photograph, and that reads as a rendering fault rather than as the end of the picture.
 
-<sub>`ui/dr-ui/ui/app.slint:2224`</sub>
+<sub>`ui/dr-ui/ui/app.slint:2002`</sub>
 
 ### Take back the last change
 
@@ -53,7 +44,7 @@ Only once there is something outside the viewport to reach, which is why the cur
 
 A whole drag is one step, so undo takes back a decision rather than a frame of a gesture. The list is there because arriving six steps back costs what arriving from one does.
 
-<sub>`ui/dr-ui/ui/app.slint:2391`</sub>
+<sub>`ui/dr-ui/ui/app.slint:2169`</sub>
 
 ### Do it again after taking it back
 
@@ -61,7 +52,7 @@ A whole drag is one step, so undo takes back a decision rather than a frame of a
 - **Pointer** — Click it, or press Redo in the History header
 - **Keyboard** — Ctrl+Shift+Z
 
-<sub>`ui/dr-ui/ui/app.slint:2404`</sub>
+<sub>`ui/dr-ui/ui/app.slint:2182`</sub>
 
 ### Copy the settings from this photograph
 
@@ -71,7 +62,7 @@ A whole drag is one step, so undo takes back a decision rather than a frame of a
 
 The panel is the copy that has to work: a tablet has no modifier key to hold and no menu bar to hang the action from. The shortcut is an accelerator for a control that is on screen either way.
 
-<sub>`ui/dr-ui/ui/app.slint:2437`</sub>
+<sub>`ui/dr-ui/ui/app.slint:2215`</sub>
 
 ### Paste the settings onto this photograph
 
@@ -81,7 +72,7 @@ The panel is the copy that has to work: a tablet has no modifier key to hold and
 
 The button names what would be pasted — "3 adjustments", and whether the crop is coming with it — which the shortcut cannot say. Both paste the same scope.
 
-<sub>`ui/dr-ui/ui/app.slint:2449`</sub>
+<sub>`ui/dr-ui/ui/app.slint:2227`</sub>
 
 ### Change which group of adjustments is on screen
 
@@ -91,7 +82,7 @@ The button names what would be pasted — "3 adjustments", and whether the crop
 
 The groups are whatever the operation set declares itself to be about, so there are as many as the pipeline has and no key can be assigned to one of them by name. Stepping is the binding that survives a node being added.
 
-<sub>`ui/dr-ui/ui/app.slint:2477`</sub>
+<sub>`ui/dr-ui/ui/app.slint:2255`</sub>
 
 ### Look at the photograph at 1:1
 
@@ -101,7 +92,7 @@ The groups are whatever the operation set declares itself to be about, so there
 
 Noise reduction and capture sharpening are judgements about single pixels, and a fitted view averages several of the file's into each one on screen — so the frame looks softer than it is and the correction goes too far. The point and the magnification survive opening the next photograph, which is what makes checking the same eye across forty portraits forty keystrokes rather than forty pans.
 
-<sub>`ui/dr-ui/ui/app.slint:2512`</sub>
+<sub>`ui/dr-ui/ui/app.slint:2290`</sub>
 
 ### Move to the next or previous photograph
 
@@ -111,7 +102,7 @@ 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:2542`</sub>
+<sub>`ui/dr-ui/ui/app.slint:2320`</sub>
 
 ### See the photograph before you edited it
 
@@ -121,7 +112,7 @@ 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:2666`</sub>
+<sub>`ui/dr-ui/ui/app.slint:2444`</sub>
 
 ### Put one control back to its default
 
@@ -139,7 +130,7 @@ The column is 280px wide and the colour mixer alone puts thirty-six of these in
 
 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.
 
-<sub>`ui/dr-ui/ui/masks.slint:195`</sub>
+<sub>`ui/dr-ui/ui/masks.slint:164`</sub>
 
 ## People