feat(structure-editor): live binary preview tab — BinaryPreviewViewModel + LiveWhfmtBuffer

[abbaye]

Context

A fourth tab Binary Preview was added to the StructureEditor (commit 58b10974), completing the live interpretation pipeline for .whfmt format definitions.

What changed

• BinaryPreviewViewModel subscribes to LiveWhfmtBuffer.Changed, runs SimpleBlockInterpreter on a background thread, exposes FieldResultRow collection to a DataGrid
• BinaryPreviewTab.xaml / .xaml.cs added
• 'Load Binary…' button triggers OpenFileDialog
• Attach/Detach lifecycle wired in StructureEditor.xaml.cs OpenAsync / Close

Files affected

• Tabs/BinaryPreviewTab.xaml / .xaml.cs (new)
• ViewModels/BinaryPreviewViewModel.cs (new)
• Controls/StructureEditor.xaml / .xaml.cs

Follow-up

• Wire binary preview to the active HexEditor selection (not just a file dialog)
• Auto-refresh on format definition save (.whfmt edit → preview updates)