Add asset server untyped handles

[waynemwashuma]

Objective

Introduce untyped asset loading APIs for type-erased asset restoration and improve asset handle snapshot serialization by storing asset type IDs instead of constructors.

Solution

HandleSnapshot previously stored a constructor reference and relied on recovering the corresponding type ID during deserialization. This created an unnecessary dependency on runtime type objects and made snapshots less suitable for serialization and type-erased workflows. This changes snapshots to store the asset type ID directly and adds an untyped asset loading path to AssetServer.

Why this approach:

• Type IDs are stable runtime identifiers and easier to serialize than constructors.
• Snapshot data becomes independent of constructor references.
• Enables asset restoration when only runtime type information is available.
• Simplifies deserialization by eliminating constructor → type ID conversion.

Showcase

New type-erased loading is available:

assetServer.loadUntyped(
  typeid(Texture),
  'texture.png'
)

Migration guide

No migration required

Notes

• Existing snapshot consumers expecting constructors must be updated.
• Serialized snapshots are now more robust because they no longer depend on runtime constructor references.
• Typed asset loading remains the recommended API for most application code.
• loadUntyped() primarily exists for serialization, reflection, editor, and tooling workflows.

Checklist

• I have updated the documentation accordingly.
• I have added tests to cover my changes.