calibUvChecker

A procedural, orientation-readable UV checker for the materialLibraryKit calibration shelf. It draws a cells-per-unit checker whose light cells carry a red gradient rising with U and a green gradient rising with V (dark cells are the same colour at dark brightness), thin black grid lines, a white-ringed blue disc in the (0,0) cell, a red arrow pointing +U and a green arrow pointing +V out of that corner, and a large outlined white "F" in the middle. It is laid out like an image texture (v = 0 is the top row), so on a correctly mapped mesh the F reads upright with the marker at its top-left; a mirrored, flipped, swapped or rotated UV layout shows immediately as a backwards or upside-down F and arrows pointing the wrong way. Lit dielectric (roughness), textureless, engine-lit surface() body. Used by the uvChecker material.