Expression and operator contributions

An operator is part of the scalar World contract, not merely a parser change. Adding one requires consistent validation, dimensional-unit rules, expression evaluation, feature generation where applicable, symbolic rendering, binary bundle encoding/decoding, and tests for accepted and rejected inputs.

Current scalar expressions support finite constants and symbols, unary neg, exp, log, sin, and cos, plus binary add, sub, mul, div, and pow. Depth is bounded. Domain errors and non-finite results must surface as diagnostics rather than being converted to sentinel values.

Custom operators are not currently serializable or executable through the production bundle/CLI path. Do not add a plugin-shaped placeholder; introduce an end-to-end capability only when its execution and persistence semantics are specified and verified.