Let a mask start from a tone or a colour, not only a shape

Every local adjustment began from a shape: painted, drawn with a handle, or
found by a model. So the only way to hold back a sky was to draw a line near
where it ended, and the only way to warm skin was to paint round it — both of
which put the edit's edge where the photographer put a gesture rather than
where the picture changes. A gradient across a treeline halos, and an
adjustment traced round a face stops on the outline of a hand.

MaskSource grows two variants that select by what a pixel *is*. Luminance
carries two bounds on the perceptual tone scale plus a softness; Colour carries
an arc of hue, a range of chroma, and one softness for every edge of both. Five
floats and three, so they diff, sync and merge per field under FR-NC-9 exactly
as a gradient's geometry does — the property a stored raster has none of, and
the reason the model's coverage had to sit beside its source rather than inside
it.

The pixels are the shader's business and nowhere else's. `mask.wgsl` takes the
demosaiced source as a sixth binding and two new modes read it: decode, balance,
pull a clipped photosite back to neutral, apply the camera matrix, then weigh
the band. Nothing crosses to the CPU but the numbers and the matrix, and each
mask texel averages its own footprint in the source, so a band lands on the tone
an area is rather than on whichever texel a proxy grid happened to land on.

The photograph it measures is the one the camera recorded, before this edit. A
band over the edited result would slide out from under the edit as the edit was
made — raising the highlights would change which pixels counted as highlights,
and the slider would chase its own mask.

Feather, falloff and morphology stay off a range layer, which is what
`shapeable` already meant. All three are functions of the signed distance from
a boundary, and a range has no boundary to be at a distance from; its edge is
the softness of its own band, in the band's units. Offering them would be four
controls that move and change nothing.

docs/requirements.md

@@ -473,7 +473,6 @@ and the subject, as a develop operation with a single symmetric amount. The tran
 estimated from the photograph rather than supplied by the photographer, and every length the
 estimate depends on is stored as a fraction of the frame, so that what is judged on screen is what
 lands in the exported file (FR-DSP-1).
-
 Haze is the one degradation the tone and colour controls cannot reach, because it is spatially
 varying: a black point that clears the mountains crushes the foreground, and a contrast curve that
 clears the mountains does the same. It belongs with texture and clarity in FR-DEV-3's adjustment
@@ -481,6 +480,15 @@ set as a member of the compositional detail family — the operations whose radi
 the picture rather than of the sensor — and it runs in the same neighbourhood stage, for the same
 reason: it is defined by what the pixels around a pixel are doing.
 
+**FR-DEV-10 — Range masks.** A mask layer may select by a band over the photograph's own values —
+luminance, or colour — with soft edges, stored as bounds and softness rather than as pixels and
+rasterised on the device in the existing mask pass.
+Every mask in FR-DEV-3 begins from a shape: painted, drawn, or found by a model. A range begins
+from a property, which is what a luminosity mask has always been and what makes a local adjustment
+blend without a halo — the boundary is the picture's own, at whatever detail the picture has. It is
+also the only route to selecting by skin tone, and the prerequisite for composing a selection from
+several criteria at once.
+
 ### 3.4 Display and interaction
 
 **FR-DSP-1 — Proxy-resolution rendering.** The develop view renders at the resolution actually