Adopt LWG 4397: reject fixed-extent span from a constant-sized range that doesn't fit

[ssam18]

Implements LWG 4397 (Improve span(R&& r)), which GCC has already shipped and LWG moved to Ready in Brno. Closes #18.

What

The range constructor gains the new Mandate: when the extent is fixed and ranges::size(r) is a constant expression, it must equal the extent. Otherwise, the program is ill-formed. This rejects e.g. span<int, 42>(views::empty<int>) at compile time instead of only tripping the runtime precondition. Ranges whose size isn't a constant expression (std::vector and friends) are unaffected and still checked at run time via the existing assert (the hardened precondition).

How

The check sits in the Extent != dynamic_extent branch of the constructor, guarded by a requires probe that only fires the static_assert when ranges::size(r) is usable as a constant expression:

if constexpr (requires {
                  std::integral_constant<size_type, std::ranges::size(r)>{};
              }) {
    static_assert(std::ranges::size(r) == Extent,
                  "span extent does not match the range's constant size (LWG 4397)");
}

Realizing the Mandate requires P2280R4, the size has to be usable as a constant expression through the constructor parameter. On compilers without it (e.g. GCC 13) the requires is simply never satisfied and the check degrades to the runtime assert, with no false rejections (fixed-extent construction from a std::vector still compiles).

Tests

• Positive unit tests: matched constant extent from views::empty<int>, and fixed-extent-from-vector staying valid.
• A guarded negative-compile test: at configure time CMake try_compiles a matching probe (must compile) and a mismatching probe (must not); only when the compiler actually rejects the mismatch is a WILL_FAIL ctest wired up around it. On compilers that don't reject it, the test is skipped with a status message.

Verified locally on GCC 13.3 (test correctly skipped, full suite green) and by simulating the reject path (test enabled and passing).

[coveralls]

coverage: 100.0%. remained the same — adopt-lwg-4397 into main