Operators as first-class folders: tune each one individually

The file-explorer treatment, applied to the engine's own building blocks: every operator in a dedicated folder with its code, its md, its html workbench and its json; schemas as json files; examples and sample data; official operator data marked standard, authored or derived (from another operator or transformation); a reusable UI to execute, test, debug and visualise input, transformation and output; and visual representations of architecture and execution, js-drawn and ascii-drawn.