Object references like `Model.PrimaryPart` or `Weld.Part0` don't get built or synced

Hello,

Noticed a bug where some `.rbxm` files that I am building into a place file do not preserve their `.PrimaryPart` properties after building with `argon build` (and by extension, `argon serve` one-way syncing does not preserve this either).

After some weld constraints broke, I noticed that they too were missing their `Part0` and `Part1` properties.
This has led me to believe that any direct object references in instances with reference properties are not preserved when building with `argon`.

If you'd like a minimum viable reproduction, I'd be happy to provide one.

For context, I'm aware that https://argon.wiki/api/properties#ref says it's not yet implemented, however the https://argon.wiki/api/properties#property-type-support-chart says that refs are supported across the board.
This seems contradictory, so some clarification on whether this is a bug or not, would be great!

Thanks for all the work that's been put into Argon.
It's a fantastic tool, everything else considered.

Start by reproducing the issue with an .rbxm containing Model.PrimaryPart and Weld.Part0/Part1, then compare argon build output with argon serve one-way syncing. Inspect the build and serve entry points and the ref property support described in the linked documentation. Done means these direct object references are preserved consistently, or the documentation clearly states the supported behavior.

Written by the indexing model from the issue text.