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/lib.rs

@@ -65,7 +65,8 @@ pub use history::{Edit, Entry as HistoryEntry, History, Step};
 pub use lens::{compose_warps, ComposedWarp, LensProfile, Tca, Warp};
 pub use operation::{
     compose, compose_with_framing, Affects, ComposedShader, Helper, Invalidation, Operation,
-    OutputMode, Uniform, BASE_CURVE_POINTS, BASE_CURVE_UNIFORM_OFFSET, RESERVED_UNIFORM_FIELDS,
+    OutputMode, Uniform, BASE_CURVE_POINTS, BASE_CURVE_UNIFORM_OFFSET, CLIP_ONSET,
+    RESERVED_UNIFORM_FIELDS,
 };
 pub use preset::{LibraryParseError, NameError, Preset, PresetLibrary, Scope};
 pub use sidecar::{Sidecar, Version};