Moves the Worley/cellular-noise-based texture handlers
(--gen-texture-cobble, -marble, -metal, -leather, -sand,
-snow, -lava) into their own translation unit. Each handler
previously had its own ~33-line copy of the parseHex lambda;
all 7 copies are replaced with a single shared parseHex
helper at file scope.
Older simpler generators (gradient/noise/radial/stripes/dots/
rings/checker/brick/wood/grass/fabric) still live in main.cpp
and will be migrated in subsequent batches.
main.cpp drops 26,286 → 25,494 lines (-792). Behavior
unchanged across all 7 handlers (re-verified).
