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/dev/architecture.md

@@ -390,6 +390,18 @@ keeps a star: real light arrives through a lens and lights a patch, so its neigh
 6×6 sensor-anchored colour tile serves Bayer and X-Trans alike, every path that demosaics gets it,
 and there is no setting.
 
+**A mask layer runs inside this chain, not after it.** Its settings are offsets to the global ones
+(`mask::offset_onto`), and each operation a layer touches is composed at the operation's own place:
+the global fragment and each layer's combined fragment read the same input, and the pixel moves by
+each layer's weighted difference, `c_g + Σ wᵢ(cᵢ − c_g)`. At full weight that is the combined
+setting exactly and at zero the global result exactly, so global contrast −30 under a layer at −20
+is contrast −50 where contrast runs, never −30 now and −20 again later — which is what the layer
+chain did before 0.18.1, and how the shadows of a night shot went magenta. A photograph with no
+layers composes to the same shader byte for byte. The film is the one exception, because it is a
+rendering rather than an adjustment and cross-fading two developments is not what a region of a
+pushed negative looks like: an operation that `blends_settings` has its uniforms averaged by mask
+weight instead, and runs once (FR-DEV-3f).
+
 There is a second reduction that does not hang off the bottom of this chain. The raw histogram
 (FR-CULL-3) taps the demosaiced scene-linear texture directly — the demosaic box's output —
 because what it measures is the file rather than the render. See §5.5.