Say that a mask layer's settings add to the photograph's

fc54523 (0.18.1) stopped running a layer as a second chain after every
global operation and made its settings offsets applied at each
operation's own place, and nothing outside the code said so. The
manual still read as though a mask's slider were a setting of its own,
frame-budget.md described the masks as "a separate chain per layer",
and no document said how a layer is blended at all.

architecture.md §5.2 now says it: the offset, the blend by the layer's
weighted difference from the global result, what a photograph with no
layers composes to, and the film as the one operation whose settings
are averaged instead (6b99f67). FR-DEV-3 records the change as resolved
beside its local-adjustments bullet, frame-budget.md's note on what it
does not measure describes the cost as it now is, and the manual gives
the arithmetic in one sentence: -20 in a mask over -30 is -50 there.
The bundled manual is regenerated to match.

docs/manual/README.md

@@ -235,7 +235,10 @@ take the crop back or keep it.
 `Local` in the rail turns the column into a mask stack. `Find subjects` runs a
 segmentation model over the photograph; what it recognises appears as a list
 of categories with how much of the frame each covers. Click one and it is a
-mask — then every slider below edits only that region.
+mask — then every slider below edits only that region. A mask's slider adds to
+the photograph's own rather than repeating it: contrast −20 in the mask over
+−30 on the whole frame is −50 there, and +30 in the mask cancels the frame's
+−30 inside it.
 
 ![What the model found in an urban scene: ground, architecture, sky, vegetation](media/local-categories.png)