Feedback addressed - force push 1 moves float_to_fx16 to convert.py. Force push 2 changes the map_prop_models.order file to be generated by the map prop model constants.

Per discord discussion I'm going to rework this to use NAIX constants for prop models instead of metang ones. I'll need to PR a change to narc to generate count constants in NAIX files first. Knocking back to draft while I'm working on that.

Sorry for the extremely noisy force push. Changes include:

• Map prop models now no longer uses constants defined in a header file or in a metang generated file - they use the map prop model NAIX constants as the source of truth. This means that any references in code and in the prop model sets use NAIX constants now.
• Prop models that I had named unk_XYZ have been renamed prop_model_XYZ to make the new NAIX constants more unique and more obvious (and they align better with the other temporary names like prop_texture_set_000 and such).
• The prop model build process now uses an explicit list of files like most other build scripts in the repo. My thinking here vs. using the order file like we were before is that future modders will only need to touch the list of files in the build script (which they will one way or the other, because meson) and they can ignore the .order file entirely. Plus it's aligned with the rest of the repo this way.