feat(editor): add --gen-texture-woodgrain end-cut tree-rings

56th procedural texture: concentric annual growth rings
centered slightly off-image (at -W*0.2, H/2) so the texture
shows sweeping arcs across most of its area rather than the
bullseye --gen-texture-rings produces.

Per-ring jitter shifts the dark-band center within each
ring's annular cell, so adjacent rings don't read as a
perfect modulus pattern — mimics real annual-growth
variation. Brightness lerps from lightHex (early-wood)
to darkHex (late-wood / heartwood) with a smooth
triangular falloff at the dark-band peak.

First procedural texture to use the new cli_arg_parse
helpers for the optional spacing/seed/W/H args.

Useful for tabletops, log-end caps (chopped firewood
stumps), barrel lids, beam cross-sections, plank ends —
distinct from --gen-texture-wood which renders vertical
grain streaks for plank surfaces.

tools/editor/cli_arg_required.cpp

@@ -88,7 +88,7 @@ const char* const kArgRequired[] = {
     "--gen-texture-planks", "--gen-texture-corrugated",
     "--gen-texture-rope", "--gen-texture-caustics",
     "--gen-texture-starburst", "--gen-texture-studs",
-    "--gen-texture-moss",
+    "--gen-texture-moss", "--gen-texture-woodgrain",
     "--validate-glb", "--info-glb", "--info-glb-tree", "--info-glb-bytes",
     "--validate-jsondbc", "--check-glb-bounds", "--validate-stl",
     "--validate-png", "--validate-blp",