Implement Substitution control

[csharpfritz]

Problem

Substitution from System.Web.UI.WebControls.Substitution provides post-cache substitution in Web Forms renders dynamic content even when the page is cached.

Blazor Approach

Implement as a component that accepts a callback delegate and renders its string output. Since Blazor doesn't use output caching the same way, this becomes a simple delegate-rendering component for migration compatibility.

Key Properties:

• MethodName (string) in Web Forms, names a static method. In Blazor, we'll use a Func or EventCallback parameter instead.

HTML Output:

Renders the string output of the callback directly (no wrapper element).

Acceptance Criteria

• Substitution.razor + Substitution.razor.cs created
• Accepts a Func callback parameter
• Renders callback output directly
• bUnit tests
• Documentation explains Web FormsBlazor migration for this control

Size: Small
Milestone: M17