Fix PDF page dimensions for rendered documents

[lhr0909]

Summary

• use 72 PDF points per inch for the PDF media box instead of 300 DPI print dimensions
• keep SwiftUI layout at display DPI, but separate layout-to-PDF scaling from print-quality rasterization scaling
• add a Swift Testing regression test that renders an A4 PDF and verifies the media box is 595.28 × 841.89 points
• run swift test in CI so the regression test is exercised on pull requests

Findings

While investigating #3, I found that PDFRenderer was using pageSize.size(atDPI: .print) for the Core Graphics PDF media box. Since .print is 300 DPI, A4 becomes 2480.31 × 3507.87 PDF units. PDF viewers interpret media box units as PDF points where 1 inch = 72 points, so the physical page is shown as roughly 875 × 1237.5 mm — close to A0.

Changing only the view/display DPI does not affect that physical page size because the oversized media box is created before rendering. The fix is to use 72-point PDF units for the page box and keep print DPI as a separate rasterization-quality concern.

Fixes #3.

Validation

swift test --configuration release