feat(editor): add --gen-texture-swirl logarithmic-spiral

70th procedural texture: N-arm logarithmic spiral. Each
pixel computes its polar angle minus log(r)*spiralFactor
and tests whether that "spiral phase" falls inside any
arm's angular band (mod 2π/N).

Distinct from --gen-texture-starburst (straight rays from
center) — this is the curved-arm vortex variant. Small
spiralFactor gives loose pinwheel arms; large factor gives
tight whirlpool spirals.

Useful for magic sigils, summoning circles, ritual floor
markings, mystical-pool surfaces, mage-tower mosaic floors,
shaman-totem ground inlays. Center-based pattern (doesn't
tile seamlessly), so best used as standalone decals rather
than wall textures.

tools/editor/cli_arg_required.cpp

@@ -105,7 +105,7 @@ const char* const kArgRequired[] = {
     "--gen-texture-blueprint", "--gen-texture-rust-streaks",
     "--gen-texture-plaid", "--gen-texture-diamond-grid",
     "--gen-texture-houndstooth", "--gen-texture-chevron",
-    "--gen-texture-dunes",
+    "--gen-texture-dunes", "--gen-texture-swirl",
     "--validate-glb", "--info-glb", "--info-glb-tree", "--info-glb-bytes",
     "--validate-jsondbc", "--check-glb-bounds", "--validate-stl",
     "--validate-png", "--validate-blp",