Add message channels for handle reference counting

[waynemwashuma]

Objective

Introduce deferred asset handle reference counting through a dedicated asset channel and separate handle lifecycle management from asset storage.

Solution

The previous implementation tightly coupled Handle reference counting to the Assets resource. Cloning or dropping a handle immediately mutated asset reference counts, requiring handles to retain a direct reference to the assets container. This made handle lifecycle management difficult to decouple from asset storage and complicated serialization/snapshot workflows. This introduces an AssetChannel that queues handle lifecycle operations and processes them during the asset container update phase.

Why this approach:

• Decouples handle lifetime management from asset storage.
• Makes handles lightweight and independent of the assets container implementation.
• Provides a foundation for scene serialization and future asset remapping workflows.
• Centralizes reference count mutations into a predictable update stage.

Showcase

N/A

Migration guide

Breaking changes

Behaviour changes

Reference count updates are no longer applied immediately.

Checklist

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