Fix #522: SVG round trips keep hidden objects and data-* attributes

An SVG used as an animation rig lost its hidden alternates and their
rotation pivots: the exporter left hidden objects out (only hidden layers
could be kept), and the importer dropped data-* attributes.

- SVG export: hiddenLayers (on by default for document.save, as before)
  now keeps hidden objects too, each written display="none" on its own
  element (path, image, text or group). Exports still leave them out
  unless asked; the CLI's convert asks, as a conversion keeps the
  document.
- An object's own data: ObjectAttributes.data, the data-* attributes in
  order (ours, data-name and data-vc-*, aside). The importer keeps them on
  the objects and layers they belong to (at most 64 per element), the
  exporter writes them back, the native format saves them (missing in
  older files), and object.setProps {data: {key: value|null}} sets or
  removes them.

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

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

@@ -70,7 +70,7 @@ pub const OPTIONS: &[FormatOption] = &[
         name: "hiddenLayers",
         ty: "boolean",
         default: "false",
-        description: "keep hidden layers, not displayed (display:none); document.save sets it unless given",
+        description: "keep hidden layers and objects, not displayed (display=\"none\"); document.save sets it unless given",
     },
     FormatOption {
         name: "encoding",

crates/engine/src/cmd/object.rs

@@ -150,7 +150,7 @@ pub fn specs() -> Vec<CommandSpec> {
             "Object Properties",
             [],
             None,
-            "{ids?|id?, name?, visible?, locked?, opacity?: 0..100, blend?: \"Multiply\"…, isolate?, knockout?: \"on\"|\"off\"|\"neutral\"|bool (true = on, false = neutral), knockoutShape?: bool}",
+            "{ids?|id?, name?, visible?, locked?, opacity?: 0..100, blend?: \"Multiply\"…, isolate?, knockout?: \"on\"|\"off\"|\"neutral\"|bool (true = on, false = neutral), knockoutShape?: bool, data?: {key: \"value\" | null (removes it)} (the object's own data, SVG data-* attributes: {pivot: \"100,180\"} is data-pivot; document.node → attrs.data)}",
             has_doc,
             set_props
         ),
@@ -738,6 +738,32 @@ fn exit_isolation(s: &mut Session, _: &Value) -> Result<Value> {
     ok()
 }
 
+/// The most data entries `object.setProps` takes at once.
+const MAX_DATA: usize = 256;
+
+/// `object.setProps`'s `data`: each key (a data-* attribute's name: letters, digits, `-`, `_`,
+/// `.`) with its value, or `None` to remove it.
+fn data_param(m: &serde_json::Map<String, Value>) -> Result<Vec<(String, Option<String>)>> {
+    const C: &str = "object.setProps";
+    if m.len() > MAX_DATA {
+        return Err(bad(C, format!("data takes at most {MAX_DATA} keys at once")));
+    }
+    m.iter()
+        .map(|(k, v)| {
+            let k = k.strip_prefix("data-").unwrap_or(k);
+            if k.is_empty() || !k.chars().all(|c| c.is_ascii_alphanumeric() || matches!(c, '-' | '_' | '.')) || k == "name" || k.starts_with("vc-") {
+                return Err(bad(C, format!("data key `{k}`: letters, digits, -, _ and . (not name or vc-…)")));
+            }
+            let v = match v {
+                Value::Null => None,
+                Value::String(s) => Some(s.clone()),
+                other => Some(other.to_string()),
+            };
+            Ok((k.to_string(), v))
+        })
+        .collect()
+}
+
 fn set_props(s: &mut Session, p: &Value) -> Result<Value> {
     let ids = targets(s, p)?;
     let opacity = p.get("opacity").and_then(Value::as_f64).map(percent);
@@ -745,6 +771,11 @@ fn set_props(s: &mut Session, p: &Value) -> Result<Value> {
         Some(b) => Some(BlendMode::parse(b).ok_or_else(|| bad("object.setProps", format!("unknown blend mode `{b}`")))?),
         None => None,
     };
+    let data = match p.get("data") {
+        None | Some(Value::Null) => None,
+        Some(Value::Object(m)) => Some(data_param(m)?),
+        Some(v) => return Err(bad("object.setProps", format!("data must be an object of key: value, not {v}"))),
+    };
     let knockout = match p.get("knockout") {
         Some(v) => Some(
             Knockout::from_value(v)
@@ -779,6 +810,24 @@ fn set_props(s: &mut Session, p: &Value) -> Result<Value> {
             if let Some(v) = p.get("knockoutShape").and_then(Value::as_bool) {
                 n.knockout_shape = v;
             }
+            if let Some(changes) = &data {
+                n.edit_attrs(|a| {
+                    for (k, v) in changes {
+                        match (a.data.iter().position(|(key, _)| key == k), v) {
+                            (Some(i), Some(v)) => {
+                                if let Some(e) = a.data.get_mut(i) {
+                                    e.1 = v.clone();
+                                }
+                            }
+                            (Some(i), None) => {
+                                a.data.remove(i);
+                            }
+                            (None, Some(v)) => a.data.push((k.clone(), v.clone())),
+                            (None, None) => {}
+                        }
+                    }
+                });
+            }
         }
         Ok(())
     })?;