Function negate_split_expr 

pub fn negate_split_expr(expr: &PartitionExpr) -> Result<PartitionExpr>

Rewrites NOT(expr) into an equivalent PartitionExpr without introducing a unary NOT node.

Why this function exists:

• PartitionExpr only models binary operators.
• Cut logic needs R AND NOT(S).
• We therefore rewrite NOT(S) into an equivalent binary-expression tree.

Rewrite rules:

• Atomic comparisons:
  • = <-> !=
  • < <-> >=
  • <= <-> >
  • > <-> <=
  • >= <-> <
• Boolean composition:
  • NOT(A AND B) => NOT(A) OR NOT(B)
  • NOT(A OR B) => NOT(A) AND NOT(B)

Failure behavior:

• For AND/OR, both sides must be Operand::Expr; otherwise returns NoExprOperand.
• Any unsupported shape bubbles up as an error and the caller degrades to no-split.