The formula text, without a leading =.
The literal's value, or undefined when the text is anything else — a reference, a
function call, an expression, or text that does not lex at all.
Backed by the calculation compiler's own lexer and literal actions (canonical locale: .
decimal, English booleans, canonical error labels), so what counts as a literal here is exactly
what counts as one in a formula. The IO borders use this to fold OOXML <formula> operands
(cellIs / data-validation bounds) into condition params — a parallel grammar that drifted
from the compiler's would corrupt exactly the operands it was meant to preserve.
Parses formula text that is exactly ONE literal — a number (optionally signed), a quoted string,
TRUE/FALSE, or an error label — to its value.