Browser-based plush spider with soft-body physics and combable fur

Build "Plush Spider" — Material Studies No. 015. Format and style: - A single self-contained HTML file on native WebGPU, fully procedural (no assets, no libraries). - The same editorial style as the series: serif masthead, side specimen panel, live readouts, "Inside the experiment" notes. The specimen: a cute plush spider sitting on a table. - The body is one signed distance field: - a big round abdomen with knitted-looking stripes; - a smaller head-and-chest tucked into it, with two fuzzy cheeks; - a paler underside. - The face: - two big glossy bead eyes on white felt discs; - a row of four tiny bead eyes above them; - blushing cheeks, an embroidered smile and two little felt fangs. - Eight posable plush legs, four a side, sewn on along the chest. They are striped like socks, end in pale little feet, and have a wire inside. Physics — body: - XPBD soft body on a hex-cell lattice: co-rotational shape matching plus tetrahedral volume, 60 Hz, 6 substeps. - Gravity and floor friction. The seat keeps its footing. - The toy slowly rights itself toward the heading it last walked (posture with a yaw). - The abdomen breathes: its cells' rest shapes and rest volumes swell. Physics — legs: - Each leg is an XPBD rod chain from a hip embedded in the body lattice to the toe. - A blended pose in the body's frame (sit, stand, hang, curl, wave) pulls each point toward its target: firmly at the hip, less toward the toe. So the legs hold their shape but sag and swing. - The wire is plastic: when the hand bends a leg past its give, the pose itself takes the new shape and the leg stays bent until "Straighten legs". - Spheres riding the body keep the legs off it. Toes grip the table. The finger pushes the legs aside. - Pulling a leg further than it reaches tows the spider by that hip. Behaviour: - Walking: - the body rises onto its legs with a height controller; - it is led toward a goal with a weak pull and turns to face it; - toes are drawn to footholds on the table; - an alternating four-and-four gait lifts the set that has fallen furthest behind and sets it down ahead, with a little arc; - it wanders on its own now and then. - Curl up (button or a finger poke): the legs hug up over its back, the spider rolls away from the touch as a ball (no footing, no rolling resistance), then unrolls and rights itself. - Hang on silk: - a springy thread that only pulls, from an anchor high above to a patch of the back; - it is reeled up off the table, swings when pushed, and its legs paddle the air; - pressing again lowers it back to the table. - Wave: one front leg is raised and wiggles. - With the Finger tool, the spider scuttles over to the finger. Rendering: - Shell fur: base plus N instanced shells, alpha-to-coverage under 4× MSAA. - The fur lie is carried by the deformation gradient, with a damped-spring tip lag. - Kajiya–Kay highlights, velvet sheen, a comb tool. - Regions painted in rest space: abdomen stripes, pale undersi