Adds heterogeneous arithmetic operators (+ - * / %) to `detail::safe_int_binary_ops`,
following the same idiom already used for `==` and `<=>`. The non-template homogeneous
inline-friends inside `safe_int<T>` keep winning same-T calls; the new template friends
in the base unambiguously handle the T != U calls without changing the converting-ctor
semantics. Resolves the ambiguity that triggered C2666 on `safe_int<int> + safe_int<long>`
when `int` and `long` have equal width (Windows).
Also:
* `mul_overflows<T>` now gates on `integer_rep_width_v<int128_t>` so MSVC's synthetic
`double_width_int<int64_t>` counts as a usable wider type for the overflow check.
* Replace the platform-dependent `safe_int<long>` widening test with a portable
`long long` variant — on Windows the original test's RHS
`static_cast<long>(INT_MAX) + 1L` is itself constexpr-invalid 32-bit overflow.
* Add convertibility coverage tests for `safe_int<T> op raw_U` mixing short / int /
long / uint8_t to lock down that the `safe_int op integral` overload wins by exact
match over the converting ctor on every platform.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
Mateusz Pusz