Function shift_formula_references 

pub fn shift_formula_references(formula: &str, op: ShiftOperation) -> String

Shift cell references in a formula when rows/cols are inserted/deleted. Returns the updated formula string.

Rules:

• Insert row at R: refs to row >= R become row + 1
• Delete row at R: refs to row > R become row - 1; row == R becomes #REF!
• Same logic for columns