hud and menus.
needs getstat* builtins, drawpic, drawfill, drawstring, various string manipulation builtins, some basic maths builtins (ie: floor), spawn+remove (for memory management rather than anything else, optional), and some entry points. job done.
draw the 3d view as normal first, stub out the 3d rendering builtins, and hope the csqc doesn't care. job done. skip any builtins that deal with any entity fields too. you're good to go.
any networking needed by a basic hud mod can be achieved through stats and stuffcmds alone.
even rmqe has more features than this with its csqc.
most csqc mods start out as a hud in some way.
I'd suggest adding/stubbing the functionality needed for my menusys stuff, if only because it uses enough engine features to be a reasonable test of everything, without going too deeply into 3d/networking stuff. I don't think it'll work in csqcwinquake straight off, you'll need to add a couple of stubs for mouse cursor modes iirc.
you can get some weird version of it from
http://triptohell.info/moodles/junk/pourtall.pk3 (source inside), extract only the csprogs.dat (not the progs.dat) and it'll just give you a new menu+hud with no other visible changes (like portal guns). that's the theory, anyway.
you can probably find a more basic mod elsewhere - one that wasn't written by someone who knows the names of all the various obscure builtins, but I think its still useful, if only for the fact that its reasonably complete.
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