[backfill, May 20] Quaternion orbit camera, TAB panel, select linked. Replaced euler camera with quaternion to eliminate gimbal lock — turntable yaw reversal when upside-down, exponential damping via slerp, pivot tracking on selection centroid. NanoVG panel with color sliders, toggles, camera/selection info. BFS select-linked (L key) floods through mesh connectivity. Five review fixes: hit map indexing, undo safety, pivot damping, select-linked fallback, panel drag leak.