M4.55: File Info with author, description, keyword chips, rating and copyright, written to SVG, PDF and PNG

file.info takes and reports the title, author, author title, description,
keywords, rating, copyright status, notice and URL in one undo step, plus
the read-only created (file.new) and modified (every save to a file) dates;
junk values are refused. Document.metadata (DocMetadata) round-trips
natively. PDF export writes author, subject and keywords to the info
dictionary and XMP, PNG export writes tEXt/iTXt chunks, SVG export writes
<desc> and, with the metadata option, every field as Dublin Core. The File
Info dialog (fileInfo, ui.fileInfoDialog) edits it with keyword chips and
star ratings.

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

crates/render/src/encode/png.rs

@@ -84,6 +84,44 @@ pub fn encode(rgba: &[u8], width: u32, height: u32, o: &PngOptions) -> Result<Ve
     Ok(out)
 }
 
+/// `png` (a PNG file) with a text chunk per `(keyword, text)` right after its header: `tEXt`, or
+/// uncompressed `iTXt` (UTF-8) for text beyond Latin-1. Keywords are 1–79 printable ASCII
+/// characters (others are skipped). Unchanged when it doesn't start with a PNG header.
+pub fn with_text(png: Vec<u8>, entries: &[(&str, Cow<str>)]) -> Vec<u8> {
+    // The signature and the IHDR chunk (length, type, 13 data bytes, CRC), which comes first.
+    const HEADER: usize = 8 + 12 + 13;
+    if entries.is_empty() || !png.starts_with(SIGNATURE) || png.get(12..16) != Some(b"IHDR".as_slice()) || png.len() < HEADER {
+        return png;
+    }
+    let mut out = Vec::with_capacity(png.len() + entries.iter().map(|(k, v)| k.len() + v.len() + 20).sum::<usize>());
+    out.extend_from_slice(&png[..HEADER]);
+    for (keyword, text) in entries {
+        if keyword.is_empty() || keyword.len() > 79 || !keyword.bytes().all(|b| (32..=126).contains(&b)) {
+            continue;
+        }
+        let mut data = keyword.as_bytes().to_vec();
+        data.push(0);
+        let latin1: Option<Vec<u8>> = text.chars().map(|c| u8::try_from(u32::from(c)).ok()).collect();
+        let ty = match latin1 {
+            Some(bytes) => {
+                data.extend(bytes);
+                b"tEXt"
+            }
+            None => {
+                // Uncompressed, no language tag, no translated keyword.
+                data.extend_from_slice(&[0, 0, 0, 0]);
+                data.extend_from_slice(text.as_bytes());
+                b"iTXt"
+            }
+        };
+        if u32::try_from(data.len()).is_ok() {
+            chunk(&mut out, ty, &data);
+        }
+    }
+    out.extend_from_slice(&png[HEADER..]);
+    out
+}
+
 /// Append one chunk: length, type, data, CRC of type and data.
 fn chunk(out: &mut Vec<u8>, ty: &[u8; 4], data: &[u8]) {
     out.extend_from_slice(&(data.len() as u32).to_be_bytes());