CLI: convert and run refuse output formats DeckCraft can't write (#715)

convert, run --save and run --export picked the format from the output
extension and fell back to native .deckcraft bytes for any extension
they didn't know, so out.odp, out.gif, out.svg and out.outline were
zips with exit status 0. They now fail before writing anything, with
the extension and the supported ones in the error.

crates/engine/src/cmd/file.rs

@@ -182,6 +182,17 @@ pub fn pdf_bytes(doc: &Presentation, p: &Value) -> Result<Vec<u8>> {
     deckcraft_pdf::export(doc, &pdf_options(p, doc.slides.len())?).map_err(|e| EngineError::Other(e.to_string()))
 }
 
+/// The output extensions [`format_for_path`] maps to a writer of that format.
+pub const WRITABLE_EXTENSIONS: &[&str] = &["deckcraft", "pptx", "potx", "ppsx", "pdf", "png", "jpg", "jpeg", "txt", "md"];
+
+/// The format a file named `path` is written in, or `None` when its extension names no format
+/// DeckCraft writes (`.odp`, `.gif`, `.svg`, no extension …): [`format_for_path`] would save
+/// those as native `.deckcraft` bytes under the wrong name.
+pub fn writable_format_for_path(path: &str) -> Option<&'static str> {
+    let ext = std::path::Path::new(path).extension()?.to_str()?.to_ascii_lowercase();
+    WRITABLE_EXTENSIONS.contains(&ext.as_str()).then(|| format_for_path(path))
+}
+
 pub fn format_for_path(path: &str) -> &'static str {
     let l = path.to_ascii_lowercase();
     if l.ends_with(".pptx") || l.ends_with(".potx") || l.ends_with(".ppsx") {