Replaced the simple swept-capsule brush with 48 Position-Based Dynamics elastic guide rods running entirely in WGSL compute shaders. Each bristle has distance constraints, bending rigidity, Coulomb friction against the paper surface, and capillary clumping when wet. The brush fans out under pressure, pinches on turnarounds, and flicks on lift-off with real inertia. Three brush types: Maru-fude (round calligraphy), Menso (fine liner), and Hake (broad wash). The difference is visceral — strokes went from "digital stamp" to "I can feel the paper" in one refactor. Also added a procedural Web Audio soundscape: bamboo water drops, singing bowls, and paper friction acoustics that respond to brush pressure.