Add unit tests for VirtualBinderFields + BinderFieldsBody

[VPDPersonal]

The Aspid.MVVM.Generators.Tests/ project is currently empty (no .cs files since the Global rename commit). The new auto-binder-fields pipeline introduced in PR #13 (VirtualBinderFields collector + BinderFieldsBody emitter) ships without regression coverage. This issue tracks adding it.

Required test cases

• AutoBinderFields default-on: [View] over a MonoBehaviour partial that implements IView<TVm> where TVm has at least one [Bind] member not already declared on the View — generated <View>.BinderFields.g.cs contains a _<name>: MonoBinder[] field decorated with [RequireBinder(typeof(<expected>))] and [SerializeField].
• AutoBinderFields = false: same input, [View(AutoBinderFields = false)] — no BinderFields.g.cs partial emitted.
• ScriptableObject base: View deriving from UnityEngine.ScriptableObject — no BinderFields.g.cs partial regardless of AutoBinderFields value.
• Skip already-declared: if the View already declares private MonoBinder[] _foo; for Foo (or inherits a binder field for it from a base [View]), it is not duplicated as a virtual field.
• [Header] propagation: VM member tagged [UnityEngine.Header("X")] → generated field decorated with [Header("X")].
• HeaderGroup propagation (single): VM member tagged [Aspid.MVVM.HeaderGroup("G")] → generated field decorated with [Aspid.MVVM.HeaderGroup("G")].
• HeaderGroupStart / HeaderGroupEnd range: VM members between [HeaderGroupStart("G")] and [HeaderGroupEnd] → all generated fields in that range carry [HeaderGroup("G")]; the field right after HeaderGroupEnd does not.
• RequireBinder types: when a VM member has multiple bindable shapes (e.g. IBindableMember<int> + IBindableMember<int?>), the generated [RequireBinder] lists each required type once, nullable suffix stripped.

Infrastructure prerequisites

The test project is xUnit + Microsoft.CodeAnalysis.CSharp.SourceGenerators.Testing.XUnit but has no source files. Setting up tests requires:

• A helper that builds a CSharpCompilation referencing Aspid.MVVM + UnityEngine stub types (ViewAttribute with AutoBinderFields property, IView<T>, IBindableMember<T>, BindAttribute, MonoBinder, IBinder, HeaderGroupAttribute / HeaderGroupStartAttribute / HeaderGroupEndAttribute, RequireBinderAttribute, UnityEngine.MonoBehaviour, SerializeField, Header, ScriptableObject).
• A CSharpGeneratorDriver wrapper that runs ViewGenerator on a given source string and exposes runResult.Results[0].GeneratedSources for assertions.

Out of scope here

• Initialization-pipeline / InitializeBody regression — covered separately if needed.
• Full BindableMembersFactory coverage — only the subset reachable via VirtualBinderFields.Collect matters for this issue.