Refuse a clipped highlight as a neutral

Sampling the overcast sky on a Canon 6D frame set tint to -100 and
temperature to -15 for a patch the canvas showed as pure white. A clipped
photosite is sensor white, not a colour: every channel stopped counting,
so what the tap hands back is the as-shot multipliers themselves, which
are strongly magenta, and the solver dutifully drove green to its stop.
The display shader already fades such a pixel to a neutral of the same
brightness before any operation runs, so the picker was balancing against
something the photographer could not see.

The probe now refuses a sample with any channel at or above the onset the
shader fades from, the way the solver already refuses black. The threshold
is one constant, CLIP_ONSET, formatted into the shader and read by the
probe, so the two cannot drift apart.

core/dr-pipeline/src/neutral.rs

@@ -69,11 +69,13 @@ const ROUNDS: u32 = 3;
 
 /// Below this a channel carries no ratio worth balancing.
 ///
-/// A sample in the deep shadows, or one taken on a blown highlight where a
-/// channel has already clipped to nothing, has no white balance in it: the
-/// logarithms below would run away and the picker would slam a slider to its
-/// stop. Refusing is the honest answer, and the caller reports that the point
-/// was not usable rather than moving the photograph.
+/// A sample in the deep shadows has no white balance in it: the logarithms
+/// below would run away and the picker would slam a slider to its stop.
+/// Refusing is the honest answer, and the caller reports that the point was
+/// not usable rather than moving the photograph. The other end — a blown
+/// highlight, where every channel has stopped counting — is refused by the
+/// caller before the sample is taken, because only the caller can see the
+/// sensor value; see [`crate::operation::CLIP_ONSET`].
 const FLOOR: f32 = 1e-4;
 
 /// TRACES: FR-DEV-3