Add a report of missing materials

When loading a large scene with a bunch of materials - Unity will spam the console with material names that are missing but it's hard to gauge what needs to be fixed (i.e. 30+ messages of the same material repeated gets annoying and other missing materials may be hidden in that spam).

Instead - there should be some sort of window where we can resolve missing materials. Similar to the Material Overrides table (or an extension that we can toggle for it) - it'll list out everything that's been mapped so far and list out everything that can't be mapped so a developer can go through the table and manually assign Unity materials into each missing slot, or fix the lookup path, etc.

And of course, as assignments change, it should update everything in the scene using that material.

Start by tracing the Material Overrides table and the missing-material lookup that produces repeated Unity console messages. Define how the proposed window lists mapped and unmapped materials, permits manual assignments or lookup-path fixes, and propagates changes to scene users; completion should include updates as assignments change.