feat(editor): add --gen-texture-snowflake 6-fold ice crystal

84th procedural texture: 6-fold symmetric snowflake stamp
tiled per cell. Built by computing polar (r, theta) from
the cell center and folding theta into a [0, pi/6] wedge,
so a single arm-shape definition replicates 12 times via
mirror + 60-degree rotation.

Each arm is a thin sliver (sin-based perpendicular distance
test) thickened at two perpendicular knobs at r = 0.40 and
0.70 of the cell-half radius — the knobs provide the
classic "branched ice crystal" silhouette without needing
a separate per-arm subdivision pass.

A small filled center dot anchors the motif at small cell
sizes where the knobs vanish.

Useful for: arctic / winter zones, frost spell effects,
frost-mage themed gear icons, holiday-event decoration,
crystal-shrine backdrops, snowstorm overlays.

Distinct from --gen-texture-snow (random tiny dots, not a
patterned crystal) and --gen-texture-frost (spider-web
crackle, not radial). The first 6-fold-symmetric texture in
the catalogue.

tools/editor/cli_arg_required.cpp

@@ -142,7 +142,7 @@ const char* const kArgRequired[] = {
     "--gen-texture-scratched-metal", "--gen-texture-crackle",
     "--gen-texture-star", "--gen-texture-halftone",
     "--gen-texture-bayer", "--gen-texture-moon",
-    "--gen-texture-damask",
+    "--gen-texture-damask", "--gen-texture-snowflake",
     "--validate-glb", "--info-glb", "--info-glb-tree", "--info-glb-bytes",
     "--validate-jsondbc", "--check-glb-bounds", "--validate-stl",
     "--validate-png", "--validate-blp",