float_ops::divmod, mod_, and floordiv each carried their own conversion
from Rust's dividend-sign `%` to CPython's divisor-sign convention. Both
divmod and mod_ mishandled the zero-remainder case where the dividend
is a non-zero exact multiple of the divisor (e.g. divmod(6.0, -3.0),
6.0 % -3.0): the sign-correction branch fired on a zero remainder and
produced (-3.0, -3.0) and -3.0 respectively, violating the magnitude
invariant 0 <= abs(r) < abs(b). divmod also leaked the wrong signed-
zero quotient when the true quotient was zero (divmod(-1.0, -2.0)
returned (-0.0, -1.0) instead of (+0.0, -1.0)).
These are independent bugs in two functions, but both come from the
same root cause: zero-remainder needs a separate path from the sign-
correction branch.
Mirror CPython's `_float_div_mod` (Objects/floatobject.c) by making
divmod the canonical implementation and turning mod_ and floordiv into
thin wrappers. divmod(a, b) == (a // b, a % b) now holds by
construction.
Closes#7722
Changjoon