Implements Task 4 of the drag-drop spine: ToolbarController now satisfies
IItemListDragHandler. The ctor loop wires RegisterDragHandler(this) on every
slot list and stamps Cell.SlotIndex + Cell.SourceKind=ShortcutBar, matching
retail's gmToolbarUI::PostInit/RegisterItemListDragHandler pattern. OnDragOver
accepts any non-empty payload (TS-33 stub; eligibility gate is Stream B.2).
HandleDropRelease logs and returns (no AddShortcut 0x019C / RemoveShortcut
0x019D wire yet). Three new B.1 xUnit tests cover handler registration, accept,
and inert-stub semantics. All 490 app tests pass.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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