hdrChart post-process shader

The synthetic HDR chart: a picture whose every pixel holds light its properties and its position state, drawn in place of the scene. Registered first in the chain (postMetrics.chart.show, priority -1000), it is the input the display transform and every effect after it read, so they are measured against known light with no lighting, geometry or material in between. @builtin::modules.postMetrics computes the same light for any pixel on the CPU (chart.light) and builds the picture a display curve should present for it (chart.reference).

Property	Meaning
pattern	0 ramp, 1 grey wedge, 2 ColorChecker, 3 colour wedge over its grey.
low, high	The ramp's light at the left and right edges.
evMin, evMax	The wedges' leftmost and rightmost stops; each column is middle grey (0.18) at a whole stop.
gain	The ColorChecker's light, as a multiple of its reflectance.
color	The ramp's tint, and the colour wedge's saturated patch (scaled to its column's grey luminance).
dither	Triangular noise of that many code values either way, added in display-encoded sRGB.

The effect declares space: light.