`arboard::set_image` does not preserve DPI (Word interprets pixels as 96 DPI)

[Mq-b]

Description

• Writing a raw pixel buffer with arboard::set_image does not include DPI/pHYs metadata.
• Result: a 591 px grayscale image (generated at 300 DPI for a 5 cm width) is interpreted by Word as 96 DPI and pastes at the wrong physical size (~15.6 cm).
• If a PNG containing pHYs/DPI is written to the clipboard, Word respects the metadata and the size is correct.

Steps to reproduce

1. Create a 591 px wide image intended to be 5 cm at 300 DPI.
2. Copy it to the clipboard with arboard::set_image(ImageData { ... }).
3. Paste into Microsoft Word — Word treats it as 96 PPI, so it appears ~15.6 cm.
4. If you instead write a PNG with pHYs/DPI metadata to the clipboard, Word reads it as 300 PPI and the image is correctly 5 cm.

Expected

• Ability to preserve/declare DPI when writing images to the clipboard, or ability to write image/png or image/tiff bytes (with pHYs/DPI) to the clipboard so receivers restore physical size correctly.

Simple suggestion

• Add set_bytes_with_format(format: &str, bytes: &[u8]) to allow writing PNG/TIFF with pHYs;
• Or add an optional dpi field to set_image so backends write clipboard representations that include DPI.