Add SDL_net support

[PaulMDemers]

Summary

Adds SDL_net support to nxdk as an SDL2 companion library, backed by nxdk’s existing lwIP network stack.

This adds SDL_net as a submodule, builds it into libSDL2_net.lib when NXDK_SDL = y, exposes a pkg-config file, and includes a small smoke-test sample that resolves localhost and opens a UDP socket.

Changes

• Add lib/sdl/SDL_net submodule
• Add nxdk/lwIP compatibility patch in the SDL_net fork
• Build libSDL2_net.lib from SDL_net sources
• Add lib/pkgconfig/SDL2_net.pc
• Add samples/sdl_net
• Update README SDL library summary

Verification

Built successfully on Windows/MSYS2 with:

make -C samples/sdl_net

This produced:

• lib/libSDL2_net.lib
• samples/sdl_net/bin/default.xbe
• samples/sdl_net/nxdk sample - SDL2_net.iso

Notes

The SDL_net submodule points at PaulMDemers/SDL_net, branch nxdk-sdl-net, which is based on SDL_net 2.0.1 with minimal nxdk compatibility changes for lwIP sockets.

[JayFoxRox]

I don't think this should be merged. See reasons in #601

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I had a port of enet at some point, and there was work towards a dummy winsock (based on lwip) which was enough for enet (I believe). @thrimbor also had a winsock impl. It should probably be extended to support the upstream SDL_net without changes, too.

Such a winsock impl would be a much better fit for core nxdk.