Draw slides on the board as frames, which Export takes as they are

[marcosqlbi]

Why

Phase E5 of docs/export.md, on top of #95 (base is the E4 branch; retarget to main once it merges). The automatic cut serves a board drawn without a deck in mind. Someone preparing a board for a deck wants to say where the slides are, and on a whiteboard the way to say it is to draw a rectangle. Decision 27 is the summary.

What it does

• View → Frame (mnemonic A) adds a frame the size of the visible board, titled "Slide n", and selects it. Drag its edge to move it, the handle to resize it (aspect kept, so it stays slide-shaped), F2 to rename it, Delete to remove it, Bring to front and Send to back to reorder it, which under Drawing order is also the slide order.
• A frame is selected by its edge or its title tab, never by its inside, so everything in it stays reachable to the pen and the mouse. Moving a frame moves nothing inside it: it is a label over content, read at export time.
• A frame is not a container. It links no strokes and takes no part in the single-container test, so a frame drawn around a picture does not stop ink from linking to the picture.
• Frames win in the partitioner: whatever sits inside one is that slide, with the frame's rectangle and title; frames come first; the rest of the board is cut as before. Frames appear in no export picture, no overview, and no preview.png.
• The archive version moves to 6 only for a board that holds a frame. A board without one is still written as version 5 and keeps opening in every release since format 5.

Where the code is

• Core/Model: FrameBoardObject with its edge-and-tab hit test; BoardDocument.HitTestTopContainer takes the zoom and reaches frames by their edge; BoardArchive writes and reads frame, choosing the version per board.
• Core/Export/BoardPartitioner: frames first.
• SQLBI.Whiteboard: BoardSurface draws frames over everything when asked and the rasterizer and preview renderer ask it not to; MainWindow gains the command, the rename, and frame cases in the gesture code; FrameTitleWindow is the rename dialog.
• Smoke tests: archive round trip and version choice, partition with a frame, hit testing.

The smaller calls are under E5 in docs/export-decisions.md.

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