Add lazy loading for labeled assets

[ewmb7701]

Objective

This pull request adds labeled asset readers so that developers can lazily load labeled assets. This is useful for so-called "random access assets", such as archives.

This resolves #25517, #21758, and #21641.

Solution

An asset loader can now register a labeled asset reader to the load context of an asset. The asset server now has various load_labeled methods which can load a labeled asset using the registered labeled asset reader and standard asset loader resolution. ie, the labeled asset reader reads the bytes from the root asset, then passes those bytes to the loader.

Testing

I wrote three tests, in crates\bevy_asset\src\lib.rs: load_labeled_uses_existing_loader_and_its_settings, load_labeled_can_load_a_dependency_from_the_same_archive, and load_labeled_can_load_a_dependency_from_the_same_archive. I tested these changes on x86_64-pc-windows-msvc.

Remarks

The code was co-authored with GPT-5.6 Sol.

This is the first step for labeled asset readers; there is more work to be done, probably requiring design and planning:

• reloading
• preloading
• refactoring existing loaders with eager/ lazy loading which uses new labeled machinery
• deprecating existing labeled asset loading
• lifecycle (adopting labeled handles if root is already loading, etc)

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