Input: drag-and-drop (file drop into the window, and drag-out)

[codymullins]

Problem

Browsers accept file drops (into a file input or the page) and support dragging links/images out. Skyline has no drag-and-drop input. Listed under shell parity in tasks/NS/wp-NS-03-chrome-parity.md.

Current state

• src/Skyline/Input/InputEvents.cs — only pointer/key/text; no drag events.
• src/Skyline/IWindowBackend.cs / AppWindow.cs — no DnD seam.

What's needed

• Drag events (DragEnter/Over/Leave/Drop) with position, offered MIME types, and a drop-data accessor (file URIs + typed bytes). Optionally a drag-out source API.
• GLFW exposes a basic file-drop callback; richer DnD needs native per-platform handlers (macOS NSDraggingDestination, Win32 IDropTarget, X11 XDND / Wayland data-device).
• Optionally gate drops through the interaction tier's Transfer capability.

Acceptance criteria

• A file dropped on the window surfaces its path(s) and MIME types to a consumer. Verified on at least one platform.