[TrimmableTypeMap] Close scanner and JCW generator gaps vs legacy pipeline

[simonrozsival]

Part of #10933. Stacked on #10924 (override detection PR).

Summary

Closes gaps between the new trimmable scanner/JCW generator and the legacy Cecil-based pipeline, discovered by building the samples/HelloWorld app and deep analysis against CecilImporter.

What we can now detect

Interface method implementations without [Register]

// User code — no [Register] on OnClick:
public class MyListener : Java.Lang.Object, IOnClickListener
{
    public void OnClick (View v) { }
}

The scanner now iterates implemented interfaces and adds their [Register]'d methods to the implementing type's marshal methods.

[Export] method access modifiers

[Export ("protectedMethod")]
protected void ProtectedMethod () { }

// JCW now correctly emits:
//   protected void protectedMethod () { ... }
// Previously always emitted "public".

[ExportField] attributes

[ExportField ("VALUE")]
public string GetValue () => "hello";

// JCW now emits:
//   public java.lang.String VALUE = GetValue ();
//   public java.lang.String GetValue () { return n_GetValue (); }

Constructor super() fallback patterns

// User ctor params don't match base — falls back to super():
public CustomParamActivity (string title, int count) { super (); ... }

Already worked correctly; now has test coverage.

Covariant return type overrides

// Base: [Register ("getResult", "()Ljava/lang/Object;")]
// Override with narrower return — JCW uses base's JNI signature.
public override Java.Lang.Object GetResult () => myString;

Already worked correctly; now has test coverage.

Tests

245 unit tests pass (+4 new test files, 25 new test methods).