Fetch the photographs around the open one ahead of the step to them

Walking the photo roll was one download per frame: every step showed
"Downloading…" over an empty canvas while tens of megabytes came down,
and moving between a pair of near-identical frames paid that a dozen
times. Now, once the opened photograph has landed, the ones around it
are fetched into the originals cache while it is being looked at, so
the next step is a disk read.

A single worker serves the latest wish only, closest first and working
outwards — next, previous, next-but-one, previous-but-one… — one file
at a time. Each open replaces the wish, so a fast walk never leaves a
trail of stale downloads competing with the one being waited on. A
process-wide in-flight registry makes a click on a photograph that is
still being fetched ahead wait for that transfer and read it from disk,
rather than start a second download of the same file.

How far each side is a setting under STORAGE — Off, 2, 5, 10 or 20,
defaulting to 5 — and it is moot while "keep originals after opening"
is off, since a fetch the cache would discard on arrival is transfer
for nothing. Nothing is fetched ahead while offline. The transfers show
in the activity list while they run and are removed when they end.

docs/gestures.md

@@ -25,7 +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:1974`</sub>
+<sub>`ui/dr-ui/ui/app.slint:1980`</sub>
 
 ### Move a magnified photograph about
 
@@ -34,7 +34,7 @@ Anchored on the fingers' midpoint, and on the pointer, so the gesture reads as m
 
 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:2065`</sub>
+<sub>`ui/dr-ui/ui/app.slint:2071`</sub>
 
 ### Paint a mask by hand
 
@@ -43,7 +43,7 @@ Only once there is something outside the viewport to reach, which is why the cur
 
 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:2152`</sub>
+<sub>`ui/dr-ui/ui/app.slint:2158`</sub>
 
 ### Take back the last change
 
@@ -53,7 +53,7 @@ A model's mask stops inside a shoulder and leaks into the hair, and no single ed
 
 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:2373`</sub>
+<sub>`ui/dr-ui/ui/app.slint:2379`</sub>
 
 ### Do it again after taking it back
 
@@ -61,7 +61,7 @@ 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:2386`</sub>
+<sub>`ui/dr-ui/ui/app.slint:2392`</sub>
 
 ### Copy the settings from this photograph
 
@@ -71,7 +71,7 @@ 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:2419`</sub>
+<sub>`ui/dr-ui/ui/app.slint:2425`</sub>
 
 ### Paste the settings onto this photograph
 
@@ -81,7 +81,7 @@ 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:2431`</sub>
+<sub>`ui/dr-ui/ui/app.slint:2437`</sub>
 
 ### Change which group of adjustments is on screen
 
@@ -91,7 +91,7 @@ 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:2459`</sub>
+<sub>`ui/dr-ui/ui/app.slint:2465`</sub>
 
 ### Look at the photograph at 1:1
 
@@ -101,7 +101,7 @@ 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:2494`</sub>
+<sub>`ui/dr-ui/ui/app.slint:2500`</sub>
 
 ### Move to the next or previous photograph
 
@@ -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:2546`</sub>
+<sub>`ui/dr-ui/ui/app.slint:2552`</sub>
 
 ### See the photograph before you edited it
 
@@ -121,7 +121,7 @@ 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:2670`</sub>
+<sub>`ui/dr-ui/ui/app.slint:2676`</sub>
 
 ### Put one control back to its default