Implement ManagedReferenceBase<T> with [SerializeReference] wrapper and editor support

[xprees]

Add a new class ManagedReferenceBase (non-breaking additive change) and a serializable wrapper for inlined values using [SerializeReference] to support polymorphic managed references.

Implementation sketch:

• New file: Runtime/Base/ManagedReferenceBase.cs
  • [Serializable]
  • public class ManagedReference { [SerializeReference] public T value; }
  • public class ManagedReferenceBase : ReferenceBase {
    // Keep existing behavior; add a new public ManagedReference inlinedManaged;
    // add editor support to draw and toggle between inlinedManaged and variable.
    }
• Editor: add PropertyDrawer/CustomPropertyDrawer that handles ManagedReference wrapper and offers type picker (uses TypeCache or assembly scan to list concrete types assignable to T).
• CloningTools: extend Clone to handle managed references wrapped in ManagedReference and deep clone lists of [Serializable] types (already present but document caveats).

Testing:

• Unit tests for cloning lists of [Serializable] classes and structs.
• Playmode tests for resetting and preserving protectedDontReset flag.

Docs:

• Update README: add section "Polymorphic and complex managed types" with examples for [SerializeReference], wrapper usage, and migration instructions.