Mythic Masters: beast silhouettes round 2: builder parts for heads, feet and wings #3071

Follow-up to the beast silhouettes PR (lane B, sheets only). Part of #2943 and #2854.

Sheet-only passes (bigger head, neck, shoulders, face parts that follow, colour blocking) moved 9 of 22 beasts up a point and stopped there. Judges still say "tiny head, tube legs, no feet, flat" for the rest. Next step needs builder parts (lane A owns blender/beast.py):

• Head shapes that are not spheres: wedge, box and muzzle-with-stop heads (a skull part: ellipsoid with a brow shelf and a jaw), so a cat, a dog and a horse differ in black fill.
• Tapering limbs, hands and feet as shapes: paws, hooves, flippers and webbed feet as parts with their own radius profile (today a foot is a node, so the leg is a tube).
• Wing membranes with a bone line and a ragged edge (Sasabonsam, Snallygaster, Ropen read as flat panels).
• Fur and scale texture bands between the back and belly colour (value contrast exists now; the blend line is a straight cut).

Beasts still at 3 to 5 in the last paired run: Agropelter 4, Fur-bearing Trout 3, Hidebehind 4, Hodag 4, Kelpie 4, Leshy 5, Olgoi-Khorkhoi 3, Raiju 4, Ropen 3, Snallygaster 3, Tatzelwurm 3, Wampus Cat 4, Yule Cat 4; Fresno Nightcrawler 2 to 3 (a faceless legend needs a face idea, not a bigger head). Plus the 40 beasts not touched yet (baseline scores in the PR).

Also: feetOf uses 15% of the beast's height, so growing a head or a hood can turn a low body node into a "foot" (Flatwoods Monster); a sheet field for the foot threshold, or counting only nodes below a fixed height, would remove that trap.