Built a complete WebGPU watercolor simulation engine in a single session. MUJŌ (無常) — a meditative Japanese sumi-e painting canvas running entirely on GPU compute shaders. No Three.js, no React, no external physics libraries. Custom Navier-Stokes fluid solver, 2-flux Kubelka-Munk spectral color mixing, and Darcy porous flow through virtual paper fibers. Five authentic Nihonga mineral pigments — sumi carbon black, cinnabar vermilion, fermented indigo, yellow ochre, malachite verdigris — each with real spectral absorption/scattering tables. The ink bleeds through virtual Kozo mulberry fibers and darkens along evaporating edges. 1024×1024 simulation grid running at interactive framerates on M4 Pro.