Add expression support for `LIMIT` and `OFFSET` by storing them as
`Expr` instead of fixed integers. Constant expressions are folded with
`try_fold_to_i64`, while dynamic ones emit runtime checks, including the
new `IfNeg` opcode to clamp negative or `NULL` values to zero. The
current `build_limit_offset_expr` implementation is still naive and will
be refined in future work.
Fixes#2913Closes#2720
7e2bfd8bc1