feat(render): Renderer with LOD selection, 3.3 and 4.3+ upload paths, headless smoke test

include/plot/gl/loader.hpp

@@ -10,6 +10,9 @@ using ProcLoader = void* (*)(const char*);
 
 /// Resolves every entry point of `PLOT_GL_FUNCS` (see api.hpp) into the global
 /// table declared there, so the rest of the library can call `gl*` directly.
+/// The above-baseline entries of `PLOT_GL_OPTIONAL_FUNCS` are resolved the same
+/// way but never fail the load: a driver without them leaves those pointers
+/// null, and callers reach them only behind `caps()`.
 ///
 /// Returns false when any required name cannot be found; entries that did
 /// resolve keep their pointers, so a failed load() never breaks a table that