feat(editor): add --gen-texture-lightbeam vertical-ray gradient

74th procedural texture: vertical light-beam / sun-ray
gradient. Each pixel's brightness is the product of two
fade factors:

  • vertical: 1.0 at the bright end, vFadeFrac at the dim
    end (direction flag selects bright-top 'd' or bright-
    bottom 'u')
  • radial: 1.0 inside the bright core (within beamHalfW of
    centerline), then linearly to 0.0 at the texture edge

Two-color blend from bgHex to beamHex by combined brightness.

Useful for dust-mote sunbeams through cathedral windows,
holy radiance auras, crystal glow halos, lighthouse beams,
projector / stage-light columns, ritual-summoning effects.
First procedural texture with a directional flag (u / d) —
flexible orientation for ceiling-down vs floor-up beams.

tools/editor/cli_arg_required.cpp

@@ -109,7 +109,7 @@ const char* const kArgRequired[] = {
     "--gen-texture-houndstooth", "--gen-texture-chevron",
     "--gen-texture-dunes", "--gen-texture-swirl",
     "--gen-texture-ironbark", "--gen-texture-mold",
-    "--gen-texture-embroidery",
+    "--gen-texture-embroidery", "--gen-texture-lightbeam",
     "--validate-glb", "--info-glb", "--info-glb-tree", "--info-glb-bytes",
     "--validate-jsondbc", "--check-glb-bounds", "--validate-stl",
     "--validate-png", "--validate-blp",