Feature history on the left side, with contiguous folders (#449)

The feature history can sit in a panel on the left, with folders that hold one unbroken run of features, drag and drop as one undo step, and a resizable divider. Timeline folders save under format 17.

crates/doc/src/format.rs

@@ -91,6 +91,9 @@
 //!   edited.
 //!   A fillet's `fit` can be `taper` (the radius follows the room along each edge it doesn't
 //!   fit). Fillets with `exact` or `reduce` save as before; 0.8.0 would refuse the unknown fit.
+//!   Feature-history folders are also first saved in 17 (`timeline_groups`: stable folder id,
+//!   name, optional parent folder and feature ids). Version 16 designs have no saved timeline
+//!   folders and open with the empty default.
 //!   A fillet's `type` can be `spline` (the radius along each edge a B-spline: `start`, `knots`,
 //!   and the control radii after `radius` in `radii`). Other fillets save as before; 0.8.0 would
 //!   refuse the unknown type.
@@ -367,9 +370,43 @@ pub fn read(s: &str) -> Result<Document> {
     if d.features.len() > crate::document::MAX_FEATURES
         || d.params.len() > crate::document::MAX_PARAMS
         || d.components.len() > crate::document::MAX_FEATURES
+        || d.timeline_groups.len() > crate::document::MAX_FEATURES
+        || d.timeline_groups.iter().any(|group| group.features.len() > crate::document::MAX_FEATURES)
     {
         return Err(DocError::Invalid("document too large".into()));
     }
+    let feature_ids: std::collections::BTreeSet<_> = d.features.iter().map(|feature| feature.id).collect();
+    let mut group_ids = std::collections::BTreeSet::new();
+    let mut grouped_features = std::collections::BTreeSet::new();
+    for group in &d.timeline_groups {
+        if group.name.trim().is_empty() || group.name.len() > 128 || !group_ids.insert(group.id) {
+            return Err(DocError::Invalid("damaged design: invalid timeline folder".into()));
+        }
+        if group.parent.is_some_and(|parent| parent == group.id) {
+            return Err(DocError::Invalid("damaged design: timeline folder cannot contain itself".into()));
+        }
+        if group.features.iter().any(|feature| !feature_ids.contains(feature) || !grouped_features.insert(*feature)) {
+            return Err(DocError::Invalid("damaged design: invalid feature in a timeline folder".into()));
+        }
+    }
+    for group in &d.timeline_groups {
+        if let Some(parent) = group.parent {
+            if !group_ids.contains(&parent) {
+                return Err(DocError::Invalid("damaged design: timeline folder has an unknown parent".into()));
+            }
+            let mut seen = std::collections::BTreeSet::new();
+            let mut current = Some(parent);
+            while let Some(id) = current {
+                if !seen.insert(id) {
+                    return Err(DocError::Invalid("damaged design: cyclic timeline folders".into()));
+                }
+                current = d.timeline_groups.iter().find(|candidate| candidate.id == id).and_then(|candidate| candidate.parent);
+            }
+        }
+    }
+    if let Some(id) = crate::non_contiguous_timeline_group(&d.features, &d.timeline_groups) {
+        return Err(DocError::Invalid(format!("damaged design: timeline folder {id} is not contiguous")));
+    }
     for o in &d.occurrences {
         if !rigid(&o.transform) {
             return Err(DocError::Invalid(format!("damaged design: occurrence `{}` has an invalid placement", o.name)));