Fix #257: format lists name every format the engine reads and writes

The export tool's description, the docs' open_file and export rows and the
CLI's run/convert usage listed only some formats. The CLI usage now prints
the writable extensions next to the readable ones, both generated from the
format table (export_extensions), the export tool points to its generated
format enum, and the docs name EPS, DXF, EMF, WMF, SVGZ and templates.

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

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

@@ -581,6 +581,18 @@ pub const OPEN_EXTS: &[&str] = &[
     "eps",
 ];
 
+/// The extension that picks each writable format when exporting (the format's first; PNG-8 shares
+/// `.png` with PNG, so `.png` comes once), in [`FORMATS`] order.
+pub fn export_extensions() -> Vec<&'static str> {
+    let mut v: Vec<&'static str> = Vec::new();
+    for e in FORMATS.iter().filter(|f| f.write).filter_map(|f| f.extensions.first()) {
+        if !v.contains(e) {
+            v.push(e);
+        }
+    }
+    v
+}
+
 /// Text files: File → Place sets them as area type (Text Import Options).
 pub const TEXT_EXTS: &[&str] = &["txt"];
 

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

@@ -150,6 +150,39 @@ fn open_exts_cover_every_readable_format() {
     assert_eq!(filters.last(), Some(&("Plug-ins", crate::cmd::plugin::EXTS)), "File › Open installs plug-ins");
 }
 
+/// The extensions usage texts list for export: one per writable format, each picking that format
+/// (`.png` picks PNG; PNG-8 is asked for by `format`).
+#[test]
+fn export_extensions_name_every_writable_format() {
+    let exts = export_extensions();
+    assert_eq!(
+        exts,
+        [
+            "vectorcraft",
+            "svg",
+            "svgz",
+            "pdf",
+            "png",
+            "jpg",
+            "gif",
+            "webp",
+            "tif",
+            "bmp",
+            "vctemplate",
+            "txt",
+            "dxf",
+            "eps",
+            "emf",
+            "wmf",
+            "tga",
+            "psd"
+        ]
+    );
+    for e in exts {
+        assert!(format_for_name(&format!("x.{e}")).is_some_and(|f| f.write), "{e}");
+    }
+}
+
 #[test]
 fn formats_query_lists_readers_writers_and_options() {
     let mut s = Session::new();