Update macOS build instructions in Compiling guide

[RobKohr]

Document Homebrew setup, repo-root cmake usage, and runtime Vulkan loader discovery on Apple Silicon.

[RobKohr]

Note: after having some issues with initially building on macos, I had Cursor AI resolve the issues, and once those changes were verified, I had it update the text in this doc with what it had to do.

I am not sure what the AI policy is, but this was verified to work correctly on macos. I made sure to have it keep the verbosity low and the changes are pretty minimal.

[bjornbytes]

Thanks for the PR! Using AI for this makes it more difficult to evaluate these changes, because I don't know which problems you were running into, which changes fix the issues you were having, and which parts are potentially a result of AI hallucinations or missing context. Some examples:

• Did you need to install with homebrew to get it working? Did the AI just recommend it because lots of people use homebrew? Were there problems with the Vulkan SDK?
• Did you have to copy the Vulkan loader next to the executable? While it may resolve issues with finding the library, it's kind of a hacky workaround and I'm not sure the docs should recommend it.
• Same with DYLD_LIBRARY_PATH, you shouldn't have to set this because LÖVR explicitly searches for /usr/local/lib/libvulkan.dylib. It used to be required on older versions of LÖVR and some other Vulkan programs, but got fixed, and now it shouldn't be necessary to set it. Maybe it's only required if you install with homebrew and it puts things somewhere else? Not sure.