Add Windows endian conversions

[kungfubeaner]

Adds the Windows-specific endian conversion support needed for the Windows port.

Provides htobe32() / be32toh() using compiler byte-swap intrinsics.
Provides htole32() / le32toh() for Windows.
Keeps the existing platform-specific implementations unchanged on Linux/macOS.

This is the first part of the Windows port and is intended to be followed by separate PRs for file handling and other Windows compatibility work.

[fmang]

Thank you for the splitting! The code looks reasonable, but the documentation is a bit off the point (like an LLM’s rambling, to be frank).

I’ll look at the rest later this week.

[fmang]

The assumption that Windows is always little-endian is worth stating, but that we use the compiler’s function instead of performing the bit operations ourselves is clear by reading the code. Unless you explain why, that mention is useless. If you wish to say that MSVC provides _byteswap_ulong, you should put the comment below, in the #else block. Just saying it provides one without giving the name of the provided function is vague.