Document AssetBundle dependency references

[Abhinav-0311]

Summary

• add a section explaining how AssetBundle dependencies differ from object-level PPtr references
• describe how m_FileID, m_PathID, archives, SerializedFiles, refs, and preload dependencies relate in UnityDataTool output
• extend the analyze representation summary with archive, serialized_file, and refs context

Fixes #50.

Testing

• Not run: documentation-only change.
• git diff --check

[SkowronskiAndrew]

Thanks for writing this up — the PPtr / external-reference explanation is the right idea and a genuinely useful thing to document. I checked the new dependency section against the Unity runtime and build source, and there are a few accuracy issues around m_Dependencies and how loading actually works. Details inline. The short version:

1. m_Dependencies is not an auto-loading mechanism, and it does not tell Unity "which bundles must be loaded." At runtime it is only used to locate preload objects among bundles the app has already loaded; a dependency that isn't loaded is silently skipped. The "which bundles to load" authority is the AssetBundleManifest (for BuildPipeline.BuildAssetBundles) or the Addressables catalog — not m_Dependencies.

2. What m_Dependencies contains differs by build path: BuildPipeline.BuildAssetBundles stores AssetBundle names, but the Scriptable Build Pipeline / Addressables stores the internal SerializedFile names (the lowercased CAB-<hash> names), not .bundle filenames. So "records bundle-level dependencies by name" is only literally true for the legacy path.

3. A PPtr never contains a dependency bundle's filename — not "not usually." The external-reference entry names the target SerializedFile by its internal archive:/CAB-<hash>/... path.

Happy to help reword; suggestions inline.

[SkowronskiAndrew]

Thanks @Abhinav-0311 for your submission, and the follow up.

That helps get some pretty relevant information into the UnityDataTool docs, I learned stuff also in doing some fact checking.

I may do a bit of a follow up to try to show some actual example diagrams or snippets, but this PR is accurate and I will merge it in, thanks.