Let the shadows and the highlights carry a colour the picture never had

The colour mixer is the only chromatic control in the chain, and it can only
turn a hue that is already in the frame. Ask it for cool shadows against warm
highlights and it has nothing to take hold of: the shadows of a correctly
balanced photograph are near enough neutral that there is no band there to
turn, and a monochrome conversion hands it a picture with no hue in it at all.
Split toning is the oldest look in the book and every developer worth comparing
against ships it; there was no way to reach it from here.

So colour_grading, declared like any other node — a hue and a strength for the
shadows, the midtones and the highlights, and a global cast over the frame. It
targets a tonal range rather than a hue, which is the whole difference between
the two controls: it puts colour where none was rather than turning what it
finds. It sits at 105, after the mixer has had the last word on the colours
that are in the picture and before the detail stage.

The mechanism is one helper. Three cosines 120 degrees apart are the hue wheel
written directly as an RGB direction, and their sum is zero at every angle, so
exp2 turns them into three gains whose product is exactly one — a cast tilts
the balance without moving the level. A grade that doubled as an exposure
change is the failure that has the photographer chasing brightness with a
colour slider, and it is corrected with a control that cannot reach it. The
three tonal weights partition the scale rather than overlapping, the midtones
being whatever the two ends leave, so setting all three to one hue is exactly
the global cast and a split tone does not colour its own midtones as a side
effect of its halves meeting. Full strength is half a stop on the leading
channel, the ceiling white balance already holds itself to.

Neutral is declared rather than inferred, which is what `active:` is for. A hue
with no strength behind it is a direction with no distance, so under the
default rule nudging one would have put the node into every fused shader for a
change nobody can see. Summing the strengths is zero exactly when all four are,
and they cannot go negative to cancel each other. The opposite reading —
neutral as "nothing has been touched" — fails the other way round: red is hue
zero, so a grade toward red never moves a hue off its default and would never
have been applied at all.

It asks for a colour wheel, the widget the descriptor vocabulary has been
carrying with no operation behind it. Nothing draws one yet, and that is fine
by construction: the panel takes the first widget it implements and falls
through to sliders otherwise, so this arrives as eight ordinary controls that
work. Each parameter is named for its own range for exactly that reason — in a
flat list, four sliders called "Hue" are four controls nobody can tell apart.

FR-DEV-12 is written into requirements.md beside it. A TRACES tag naming a
requirement that is not defined there is an orphan, and the traceability gate
fails on those rather than quietly counting them. The label catalogue gets one
line for the operation's display name; the eight parameters derive correctly
and are left to.

ui/dr-ui/src/labels.rs

@@ -138,6 +138,7 @@ fn catalogued(key: &str) -> Option<&'static str> {
         "op.vibrance" => "Vibrance",
         "op.saturation" => "Saturation",
         "op.colour_mixer" => "Colour Mixer",
+        "op.colour_grading" => "Colour Grading",
         // "Sharpening" rather than what `derive` would make of the id. The id
         // says *capture* sharpening to separate it from the output sharpening
         // an export applies (FR-EXP-4), which is a distinction about where in