fix: bound the image decode transient and keep native picks on disk

Follow-up to the #1717 port:

- The downsample tier holds the decoded RGBA, the resize's vertical-pass
  intermediate (width x new height in RGBA f32) and the output at once;
  that sum now has to fit the decode budget, or the image is refused up
  front instead of overshooting it several times over. The native
  budget drops from 4 GiB to 2 GiB.
- A native IMAGE pick decodes from the picked file instead of copying
  its bytes into the session registry, so an edited file reloads and the
  bytes are not held for the session. The web keeps registering them.
- Reword a plot-preview comment.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/io/print_to_printer.rs

@@ -9,8 +9,8 @@
 //
 // The web build has no system printers: `preview_page_pdf` /
 // `preview_pages_pdf` render the same PDF bytes, rasterise them, and show
-// the pages in an in-page overlay — the way AutoCAD previews plot output on
-// screen rather than handing a file to an external viewer.
+// the pages in an in-page overlay — the plot preview stays on screen
+// rather than handing a file to an external viewer.
 
 #[cfg(not(target_arch = "wasm32"))]
 use crate::io::pdf_export;