fix: remove all USE_SDL compile flags — SDL3 always via dlopen
Removed all #ifdef USE_SDL guards, -DUSE_SDL, -lSDL3, SDL3/SDL.h
include. SDL3 code always compiles with void* types and dlsym
function pointers. No compile-time or link-time SDL dependency.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>