Draw a background grid that shows the zoom level

[marcosqlbi]

Closes #129.

A board is a white plane, and at any zoom it looks the same. Nothing on screen
says whether you are drawing at half size or at four times, so a stroke that
matched the one beside it a minute ago turns out not to, and a container dropped
next to another is a guess. This is the cue the issue asks for: a faint grid
whose spacing stays fixed in board pixels, so zooming shows the lines spreading
and then stepping coarser, and the zoom level becomes something you can see.

It is off by default, and it is an application preference rather than something a
board carries — it says how one person likes to work, and a board opened
somewhere else should not arrive with their grid. Preferences → Board →
Background grid offers Off, Lines, and Dots; View → Grid toggles between
Off and whichever of the two was last chosen, so the button never has to ask
which grid you meant. The two stay in step: choosing Dots in Preferences is what
the toggle brings back afterwards.

GridGeometry in Core.Viewport owns the arithmetic — the spacing for a zoom,
and the world positions of the lines crossing a visible rectangle — so the
surface only turns positions into screen coordinates, and the stepping is
testable without WPF. The spacing is 40 board pixels, multiplied by four while
the lines would fall closer than 12 screen pixels apart.

The grid never leaves the screen. BoardRasterizer and BoardPreviewRenderer
build their own surface and now say GridStyle = GridStyle.Off beside their
existing DrawFrames = false, rather than relying on the default, so the rule is
visible to the next person reading either file. Nothing about the board file, the
archive version, or the selection changes.

Settings go to version 19, once, as the plan says; every new field has a default,
so 18 → 19 needs no migration. No version bump and no changelog entry: that is
pull request 7.

Two readings the plan left open, taken here:

• The Preferences editor is the plain combo, not drawn swatches. The three
  choices use the existing EnumChoice, because a drawn choice needs a new
  SettingEditorKind and its own sample renderer, and the brief was not to build
  one for this. The row's prose carries what the pictures would have said.
• A dot reaches 1.5 pixels out from its intersection, rather than measuring 1.5
  across. At the grid's gray — #14000000, the same as the lines — a dot 1.5
  pixels wide covers so little of a pixel that antialiasing thins it to about
  three per cent of black, which is not there at all on a screen. At this radius
  a dot is as dark as a line pixel and still reads as subtler than the lines,
  which is what a dot grid is for. The gray is unchanged.

The dots are drawn as one geometry rather than as an ellipse each, because the
surface is redrawn on every frame of a pan and a dense grid runs to tens of
thousands of intersections.

Also worth knowing: the Grid button sits on the View row, which is not a sticky
tab, so pressing it closes the row exactly as Full screen and Canvas only do.
Turning the grid on and straight off again therefore takes two trips through the
tab. Changing that would change every View command, so it is left alone here.

The icon is Fluent UI System Icons' grid_24_regular, MIT, added to
Toolbar.xaml beside the others.

The smoke tests cover the spacing steps, the exact threshold and either side of
it, the line positions for a known rectangle, and the settings round trip and
normalization. In the WPF harness the surface is rendered at each style and
compared pixel for pixel, so a style that drew nothing would fail; a preview is
checked to be the same bytes twice and a fresh surface to start at Off, which is
where a preview's grid is decided. SQLBI_WHITEBOARD_GRID_PREVIEW=<path> writes
a sheet of the three pictures, in the manner of the Prompt harness, for anyone
who wants to see how faint faint is.

To try by hand

• Open View → Grid. The button should check — accent icon and label, tinted
  background — and a faint grid should appear behind the board.
• Zoom out slowly. The lines should spread and then snap to a coarser spacing
  rather than sliding closed, and the same going back in.
• Pan at several zooms. The grid should stay crisp and one pixel wide, with no
  shimmer and no visible cost to the pan.
• Switch to Dots in Preferences → Board. The View button should stay
  checked, and toggling it off and on again should bring dots back, not lines.
• Turn the grid on, then Export a deck and a PDF, and Ctrl+C a selection:
  none of them should carry the grid. The same for the Explorer thumbnail and the
  VS Code preview of a saved board.
• Alt+V then G should toggle it from the keyboard.
• With the grid on, check it against a highlighter stroke and a picture: it is
  meant to sit under everything.

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