abacus-add-function

How to add (or remove/rename/alias) a built-in function in the abacus expression language — math functions like sqrt/sin/hypot/atan2, variadic stats like sum/median, nullary constants like pi/e/time. Use whenever the task is to implement a new abacus call function, add a function alias, make a function multi-argument, or wire a new constant. Covers the Value method + registry two-step, per-mode…