Add IInterceptedRelatedManager opt-in seam to BindingContext/RelatedCurrencyManager

[Copilot]

On .NET Framework, ZBindingContext intercepted BindingContextHashtable.Add to swap in a custom CurrentChanged handler before the child manager was primed — so the empty-parent Everett AddNew()/CancelCurrentEdit() branch was never reached. On .NET 10 the backing store is a Dictionary, making that interception impossible. The previous RewireParentChangeHandler approach fired after construction, meaning ParentManager_CurrentItemChanged (and thus AddNew()) already ran before any rewire could occur.

This PR introduces a factory-based opt-in seam that installs a custom handler before the first prime, exactly replicating Framework ordering — with zero overhead on the default path.

BindingContext

• Added BindingContext.IInterceptedRelatedManager — public nested interface (ParentHasRows, RaiseParentCurrentItemChanged, BindToEmptyParentPlaceholder) enabling a subclass in another assembly to drive the internal RelatedCurrencyManager without reflection.
• Added protected virtual EventHandler? CreateInterceptedParentHandler(IInterceptedRelatedManager) — returns null by default; overriding activates the opt-in path.
• Added private CreateRelatedCurrencyManager(...) helper — factory delegate is only allocated when GetType() != typeof(BindingContext), guaranteeing no overhead for stock WinForms.
• Removed static RewireRelatedCurrencyManagerParent(...) (superseded).

RelatedCurrencyManager

• New overloaded constructor accepts Func<RelatedCurrencyManager, EventHandler>?; the factory is invoked before Bind() so _interceptedParentHandler is set before the first prime.
• Bind() gates on _interceptedParentHandler: default path calls ParentManager_CurrentItemChanged as before; opt-in path calls the supplied handler (bypassing the Everett AddNew() branch).
• WireParentManager/UnwireParentManager subscribe to CurrentChanged (not CurrentItemChanged) when a handler is active.
• Explicit BindingContext.IInterceptedRelatedManager implementation added.
• RewireParentChangeHandler() removed (superseded).

Usage pattern

// In a derived BindingContext (e.g. in another assembly):
protected override EventHandler? CreateInterceptedParentHandler(
    BindingContext.IInterceptedRelatedManager relatedManager)
{
    return (sender, e) =>
    {
        if (relatedManager.ParentHasRows)
            relatedManager.RaiseParentCurrentItemChanged(sender, e);
        else
            relatedManager.BindToEmptyParentPlaceholder(new TempList());
    };
}

API surface

PublicAPI.Unshipped.txt updated: RewireRelatedCurrencyManagerParent removed; IInterceptedRelatedManager interface and CreateInterceptedParentHandler virtual added.