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/frame-budget.md

@@ -325,9 +325,12 @@ without measuring them:
 Stated because §7 of [display-and-extension.md](display-and-extension.md) asks
 for it, and because each of these could move the numbers.
 
-- **Local adjustments.** The mask stack is a separate chain per layer and is not
-  in any row above. `render_masked` takes them and the fused shader addresses
-  them per layer, so a heavily masked edit costs more than `all`.
+- **Local adjustments.** Not in any row above. Since 0.18.1 a layer is no longer
+  a separate chain after the global one: each operation a layer touches runs a
+  second fragment at its own place in the chain, blended by the layer's mask
+  ([architecture.md §5.2](architecture.md)), and `render_masked` binds the mask
+  array the fused shader samples. A heavily masked edit therefore costs more
+  than `all`, by roughly one fragment per touched operation per layer.
 - **Spot repairs.** These add detail passes, and their cost is per spot.
 - **Lens corrections.** Not part of `EditGraph::default_chain` — they are built
   from a matched profile — so the `point` row does not include the warp chain.