Fix is_convertible_without_narrowing for corner cases

[saki7]

Follow-up for #54

This PR addresses corner cases mentioned in https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2026/p0870r8.html#ch6.

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Note: there's one minor case that is unfixable right now; this specific case (also mentioned in the paper) cannot be handled because GCC/Clang and MSVC(2026) currently disagree.

Although the paper's author "believes" this should return true, it is technically undecided right now; LWG decision (not LEWG decision) is needed.

STATIC_CHECK(iris::is_convertible_without_narrowing_v<std::integral_constant<int, 42>, float>);

[cppwarningnotifier]

Environment				C++23	C++26
iris	Clang	21	Debug	✅success	✅success
			Release	✅success	✅success
	GCC	14	Debug	✅success	✅success
			Release	✅success	✅success
	MSVC	2026	Debug	✅success	✅success
			Release	✅success	✅success