0.5.3

Document multi-platform support for stable native mods

Stable mods can run on Windows, macOS and Linux, because the SDK is plain
Rust source rather than the classic path's pre-compiled Windows rlibs.

Explain how platform support is decided: it is derived from the module
binaries in the package (.dll / .so / .dylib), with nothing to declare in
mod.mod_info, so a package can never advertise a platform whose binary is
missing. Data-only mods have no platform at all.

Make clear that the modder builds each platform themselves. Compilation is
portable but linking is not: a .dylib needs a Mac because Apple's SDK cannot
be redistributed, and a .so needs Linux or a cross-linker the modder installs.
Supporting Windows only is a normal outcome.

Also cover shipping several binaries in one Workshop item, the diagnostic
players see on an unsupported platform, per-platform SDK locations, the
lib-prefix rename when installing a .so/.dylib, and the Runs on: line the
uploader adds. Correct the upload skip list, which omitted target/ and the
Cargo manifests.