feat(editor): add --gen-texture-starburst radial-rays pattern

53rd procedural texture: N rays radiating from the texture
center. Each pixel computes its angle via atan2 and finds
the angular distance to the nearest ray axis (handling the
±π wrap so rays around the seam are continuous). Pixels
inside any ray's angular band get the ray color.

Brightness tapers linearly with distance from center: 1.0
at the hub, 0.4 at the texture diagonal — gives sun-rays
that fade as they extend outward instead of reading as
infinite lines.

Useful for sun motifs, holy/divine symbols, paladin
insignias, mage-robe trim, mosaic medallion centers,
shrine floor inlays, and any "radiant glory" surface.

tools/editor/cli_arg_required.cpp

@@ -86,6 +86,7 @@ const char* const kArgRequired[] = {
     "--gen-texture-knit", "--gen-texture-chainmail",
     "--gen-texture-planks", "--gen-texture-corrugated",
     "--gen-texture-rope", "--gen-texture-caustics",
+    "--gen-texture-starburst",
     "--validate-glb", "--info-glb", "--info-glb-tree", "--info-glb-bytes",
     "--validate-jsondbc", "--check-glb-bounds", "--validate-stl",
     "--validate-png", "--validate-blp",