Class AgentManagedStorage

Storage for per-agent data that requires managed types.

The Unity ECS wants component data to be unmanaged, but an agent needs to keep hold of things that cannot be: its current path, a traversal provider, user callbacks. Historically these lived in class-based IComponentData components, but those are being removed from the Entities package.

Instead each agent owns one slot in a flat array here, and refers to it through an AgentManagedRef component holding nothing but an integer.

# Threading

Reads take a single snapshot of entries and index it. Growth allocates a larger array, copies, and publishes the new reference, so a reader racing with growth keeps using the pre-growth array; that array stays alive and every slot it already contained still holds the right data.

Invariant: every write to an entry goes through one of the locked methods here, never through a snapshot the caller is holding. Growth copies entry payloads rather than aliasing them, so a write through a snapshot that growth has already superseded lands in an array nobody reads any more and is silently lost. Taking the lock re-reads entries, which is what makes a write always land in the live array.