Report logical plot size in display_data metadata

[seeM]

Addresses posit-dev/positron#15261.

Summary

Ark renders notebook plots at logical size × pixel ratio physical pixels. Previously, its display_data and update_display_data messages did not report the intended logical dimensions, so frontends could display a 2× render at twice its requested width and height.

This PR adds standard Jupyter PNG metadata containing the logical width and height:

{
  "image/png": {
    "width": "<logical width>",
    "height": "<logical height>"
  }
}

The PNG retains its higher physical resolution, while frontends—including Positron's Quarto inline-output renderer—display it at the requested logical size.

Screenshots

Before

After

QA

On a HiDPI display, enable inline output in a Quarto document and run:

#| fig-width: 4
#| fig-height: 3
plot(1:10)

The plot should:

• display at 4 × 3 inches rather than 8 × 6 inches;
• remain sharp at the display's native pixel ratio.

[lionel-]

@thomasp85 might be interested in taking a look too