142Build an Excel-like spreadsheet with editing, formulas, dependency tracking, and virtualized rendering

The heart of a spreadsheet is its calculation engine. (The virtualised grid UI is covered in #116, and the full design in #135.) Implement createSpreadsheet() returning:

• set(ref, input): set a cell ("A1", "b12") to a number ("42"), text ("Total") or a formula ("=SUM(A1:A3)*2"). An empty string clears it.
• get(ref): the computed value: a number, a string, null for empty, or an error string.
• getInput(ref): what the user typed.
• onChange(listener): after every set, call listener(changedRefs) with the refs whose value changed (sorted); returns an unsubscribe function.
• getWindow(top, left, rows, cols): a 2D array of computed values for a rectangle (what a virtualised grid asks for).
• evaluations: a counter of formula evaluations, to prove recalculation is incremental.

Formulas support numbers, cell references, + - * / with normal precedence, parentheses, unary minus, and the functions SUM, AVERAGE, MIN and MAX with ranges (A1:B3) and lists of arguments.

Rules:

• A reference to an empty cell is 0 in arithmetic. In ranges, functions ignore empty and text cells.
• Errors: #DIV/0! (division by zero), #VALUE! (text used in arithmetic), #NAME? (unknown function), #ERROR! (cannot parse), #CYCLE! (circular reference). An error in an input propagates to formulas that use it.
• Changing a cell recalculates only the cells that depend on it, each once, in dependency order.