Fix CompileFast returning wrong values for member access on an interface converted to a struct

[Copilot]

CompileFast() returned wrong results for ((Start)x).Member when x is an interface-typed value and Start is a struct. Depending on the member, it returned the struct itself, default, or a different member's value. Compile() and the object → struct case were correct.

// x is IHasId, Start : IHasId is a struct
Expression<Func<IHasId, object>> e = x => ((Start)x).Number;
e.Compile()(new Start());      // 42
e.CompileFast()(new Start());  // 0 (before this change)

• Root cause: TryEmitConvert only emitted unbox.any when the source type was exactly object (or Enum). For an interface source it fell through to the generic path, which left the boxed reference on the stack. The member access then treated that reference as a struct value or address.
• Fix: TryEmitConvert in FastExpressionCompiler.cs now also emits unbox.any when the source is an interface that the target value type implements. Interface → class conversions still go through castclass.
• Tests: new Issue553_* test in FastExpressionCompiler.IssueTests, compiled into the LightExpression variant too. It covers a string property, an int property, a field and a method call on (Start)interface. Both variants are registered in TestsRunner/Program.cs.

The fix only covers interfaces. Other non-value source types that a struct can convert from, such as ValueType, are not touched here.

• Fixes CompileFast returns wrong values when a member is accessed on an interface converted to a struct #553