Render: decode print-size rasters, and say when an image is missing from an export

The renderer decoded images with the decoder's default 512 MB allocation
limit, so a 300 ppi roll (6614 x 59055 px, 1.56 GB as RGBA) that opens and
exports to PDF fine drew nothing in a raster export - a fully transparent PNG,
exit code 0, no warning. It now decodes with the allowance opening has (2 GB,
512 MB on the web), shrinks an image past a pixmap's 65535 px side to fit
instead of dropping it, and counts any image it still can't decode: a raster
export that drew fewer images than it should says so in its warnings.

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

crates/engine/src/cmd/fileio/encode.rs

@@ -382,7 +382,13 @@ pub fn encode_all(doc: &Document, format: &str, p: &Value) -> Result<Encoded> {
 /// [`encode_all`] of `doc` as it is.
 fn encode_all_of(doc: &Document, format: &str, p: &Value) -> Result<Encoded> {
     let f = super::writable(C, Some(format), None)?;
+    let failed = vectorcraft_render::decode_failures();
     let mut enc = encode_files(doc, f, p)?;
+    // An image the renderer couldn't decode draws nothing: say so rather than write a picture
+    // that silently lacks it.
+    if f.raster && vectorcraft_render::decode_failures() > failed {
+        enc.warnings.push("an image couldn't be decoded and is missing from the picture".into());
+    }
     // Formats that draw type: missing fonts come out in the fallback font (PDF and SVG say so
     // themselves: SVG only when it outlines or embeds fonts).
     if (f.raster || matches!(f.id, "eps" | "emf" | "wmf"))

crates/engine/src/cmd/fileio/tests_raster.rs

@@ -163,3 +163,20 @@ fn an_image_opens_at_the_size_its_resolution_declares() {
         assert!((art.width() - w).abs() < 1e-6 && (art.height() - h).abs() < 1e-6, "{what}: the image fills it, {art:?}");
     }
 }
+
+#[test]
+fn a_raster_export_says_when_an_image_could_not_be_decoded() {
+    // A PNG whose header reads (so it places at its size) but whose pixel data is cut short:
+    // the renderer can't decode it and draws nothing, which the export must say.
+    let mut s = session(72.0, 36.0, 1);
+    let mut png = Vec::new();
+    image::RgbaImage::from_pixel(64, 32, image::Rgba([200, 30, 30, 255]))
+        .write_to(&mut std::io::Cursor::new(&mut png), image::ImageFormat::Png)
+        .unwrap();
+    let cut = png_chunk(&png, b"IDAT").map(|d| d.as_ptr() as usize - png.as_ptr() as usize).unwrap() + 4;
+    png.truncate(cut);
+    s.execute("file.place", &json!({"name": "cut.png", "dataBase64": vectorcraft_format::base64_encode(&png)})).unwrap();
+    let r = s.execute("document.export", &json!({"format": "png", "background": "transparent"})).unwrap();
+    let warnings = r["warnings"].as_array().cloned().unwrap_or_default();
+    assert!(warnings.iter().any(|w| w.as_str().is_some_and(|w| w.contains("couldn't be decoded"))), "{r}");
+}