Sync iccviz engine: GamutVolume, RoundTripDE, neutral-axis inking (InternationalColorConsortium#1711)

Bring the data-first visualization engine under Tools/CmdLine/IccProfilePlot/
(IccVizModel, landed in InternationalColorConsortium#1517) up to date with three feature-additive changes,
plus the caller updates they require. No refactor of the enumerate/render
surface; no change to IccProfLib or to the Mini{PDF,SVG,TIFF} writers.

Engine (IccVizModel.hpp/.cpp, IccVizMath.hpp):
- GamutVolume(): gamut volume in dE*ab^3 via boundary voxelisation + flood-fill
  through the AToB transform; scalar GamutVolumeResult with a `degenerate` flag
  so a collapsed sampling is distinguishable from a genuinely tiny gamut.
- RoundTripDE(): B2A->A2B round-trip accuracy (mean/p90/max/std dE*ab) seeded
  from in-gamut device-cube samples; scalar RoundTripResult.
- Kind::NeutralAxisInking: device colorant along the neutral axis from a
  PCS->device (BToA) table, enumerated per BToA tag. InkReversalL retired
  (enum value reserved). Plus GamutVolume accuracy work (IccProfLib CIELAB
  reuse) and CLUT-raster / new-metric hardening against malformed input.

Breaking API (internal only): Enumerate(pIcc, Order) loses its Order argument.
Enumerate(pIcc) now returns one canonical order (chromaticity, then per-tag
A2B0/1/2, B2A0/1/2, gamut, then named/colorant). Only in-tree caller updated.

iccProfileVisualize.cpp:
- Drop the Order argument at the Enumerate call site.
- Render Kind::NeutralAxisInking into the PDF report (was engine-enumerated but
  silently skipped): CreateNeutralAxesXobject (L*/%-ink axes frame) +
  renderNeutralAxisGraph (one polyline per colorant along L* 100->0, distinct
  CMYK stroke, channel-name legend) + the processLuts switch case. One page per
  PCS->device table.

The engine sources are algorithmically identical to the upstream copy; the only
deltas are the restored ICC license header on IccVizModel.cpp and an ASCII
punctuation sweep (matching the InternationalColorConsortium#1517 strict-build convention), which changes
five graph-title string literals from an em-dash to a hyphen. All translation
units build clean under -Wall -Wextra.

The old "byte-identical vs original iccProfileVisualize" PDF baseline is
intentionally superseded by the canonical page order and the chromaticity page
for device/CMYK profiles; a pinned-output regression + CTests for the two new
scalar metrics follow as a separate in-repo PR per the maintainer-owned CTest
policy. Refs InternationalColorConsortium#1711.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: colourbill-ctrl

Tools/CmdLine/IccProfilePlot/IccVizMath.hpp

@@ -62,13 +62,13 @@
  */
 
 /**
- * IccVizMath  -  small shared colour-math helpers for the visualization model.
+ * IccVizMath - small shared colour-math helpers for the visualization model.
  *
  * The pure scalar conversions the visualizations need: XYZ->xy chromaticity
  * projection and the Kang et al. planckian-locus approximation.
  *
- * They live in one header so the math  -  especially approxPlanck()'s polynomial
- * constants  -  has a single source. The functions are header-only (inline) and
+ * They live in one header so the math - especially approxPlanck()'s polynomial
+ * constants - has a single source. The functions are header-only (inline) and
  * depend only on the ICC scalar types, so the header carries no link dependency.
  */
 
@@ -85,7 +85,9 @@ struct XY {
   float y = 0.0f;
 };
 
-// 16-bit ICC XYZ (s15Fixed16 stored as integer/65535) -> CIE xy chromaticity.
+// xyFromICCXYZ - project a 16-bit ICC XYZ (s15Fixed16 stored as integer/65535) to
+// CIE xy chromaticity. Separate from the float version because the integer source
+// can't be NaN/Inf, so it skips those checks; a near-zero sum returns (0,0).
 inline XY xyFromICCXYZ(const icXYZNumber* xyz) {
   // integers, so don't have to test for NaN or Inf
   float X = xyz->X / 65535.0f;
@@ -96,7 +98,9 @@ inline XY xyFromICCXYZ(const icXYZNumber* xyz) {
   return XY{X / sum, Y / sum};
 }
 
-// Float XYZ -> CIE xy chromaticity.
+// xyFromXYZFloat - project a floating-point XYZ triple to CIE xy chromaticity; the
+// float twin of xyFromICCXYZ for values that come from a transform rather than a
+// stored tag. A near-zero (or non-positive) sum returns (0,0) to avoid dividing by ~0.
 inline XY xyFromXYZFloat(const icFloatNumber* xyz) {
   float X = xyz[0];
   float Y = xyz[1];
@@ -106,7 +110,10 @@ inline XY xyFromXYZFloat(const icFloatNumber* xyz) {
   return XY{X / sum, Y / sum};
 }
 
-// Planckian (blackbody) locus approximation in CIE xy.
+// approxPlanck - approximate the planckian (blackbody) locus in CIE xy for a colour
+// temperature `t` (Kelvin), used to draw the reference curve on the chromaticity
+// chart. Uses the Kang et al. piecewise polynomial fit rather than integrating the
+// Planck radiator, which is fast, table-free and accurate enough for a plotted guide.
 //   https://en.wikipedia.org/wiki/Planckian_locus
 //   Bongsoon Kang; Ohak Moon; Changhee Hong; Honam Lee; Bonghwan Cho;
 //   Youngsun Kim (December 2002). "Design of Advanced Color Temperature Control