Libraries panel: the list scrolls; items and groups reorder by dragging, items drag into groups

The list fills the panel down to its bottom bar and scrolls, also while a drag nears its top or
bottom edge (in the dock and in a flyout, as the Layers panel does).

Dragging in the list:
- an item (colour, text style or graphic) dropped on another of its kind lands just before or
  after it, by the pointer's half, an accent line showing where, and joins that item's group;
- dropped elsewhere on a group (its header too, even collapsed) it goes into that group, and on
  the ungrouped items out of its group, the section outlined; with every item grouped, a
  "Drop here to take it out of its group" strip shows while one is dragged;
- a group dragged by its header goes before or after another group.
Colours and graphics still drag onto art and the canvas as before.

Engine: library.moveItem takes `before` / `after` (an item of the same kind) to place the item
in its kind's order, which the panel shows; the new library.moveGroup {group, to} moves a group.
Text styles drag as PanelDrag::LibraryItem, groups as PanelDrag::LibraryGroup, and a library
colour's Paint drag names its library item, so the panel tells it from a swatch dragged in.

Tests: reordering and moving in the engine, the commands in the library fuzz test, and panel
tests that drop styles and group headers and check the list scrolls and scrolls while dragging.
Strings translated in the 23 catalogs; docs/mcp.md and the parity row updated.

crates/engine/src/cmd/library.rs

@@ -5,7 +5,8 @@
 //! one copies the selection as Copy does, and placing it pastes a copy as Paste does (its images,
 //! symbols, patterns and swatches come along; a swatch name the document gives another colour is
 //! merged into the document's). Items can be put in user-named groups, and a library can be
-//! exported to a `.vclibrary` file and imported from one (#926).
+//! exported to a `.vclibrary` file and imported from one (#926). Items and groups are kept in the
+//! order the Libraries panel shows them, which dragging them there changes.
 
 use std::collections::{HashMap, HashSet};
 
@@ -281,6 +282,25 @@ impl Library {
         }
     }
 
+    /// Move item `r` next to `to`, an item of its kind: just before it, or just after it when
+    /// `after`, in their kind's list (the order the panel shows them in).
+    fn place_next_to(&mut self, r: &ItemRef, to: &ItemRef, after: bool) {
+        fn shift<T>(list: &mut Vec<T>, key: impl Fn(&T) -> &str, from: &str, to: &str, after: bool) {
+            let Some(i) = list.iter().position(|x| key(x) == from) else { return };
+            let item = list.remove(i);
+            let j = list.iter().position(|x| key(x) == to).map_or(list.len(), |j| j + usize::from(after));
+            list.insert(j.min(list.len()), item);
+        }
+        let (from, to_key) = (r.item.as_str(), to.item.as_str());
+        match r.kind {
+            ItemKind::Color => shift(&mut self.colors, |c| c.name.as_str(), from, to_key, after),
+            ItemKind::CharStyle => shift(&mut self.char_styles, |c| c.name.as_str(), from, to_key, after),
+            ItemKind::ParaStyle => shift(&mut self.para_styles, |c| c.name.as_str(), from, to_key, after),
+            ItemKind::Graphic => shift(&mut self.graphics, |g| g.id.as_str(), from, to_key, after),
+            ItemKind::Unknown => {}
+        }
+    }
+
     /// Make a library read from a file (hand-edited, imported) safe and consistent: at most
     /// [`MAX_ITEMS`] items of each kind and [`MAX_GROUPS`] groups, names clipped to [`MAX_NAME`]
     /// characters and unique within their kind (a repeated one gets a number), sizes finite, a
@@ -552,10 +572,19 @@ pub fn specs() -> Vec<CommandSpec> {
             "Move Library Item to Group",
             [],
             None,
-            "{library?, kind, item: name (graphic: id or name), group?: name (null or none: out of its group)} put an item in a group (out of the one it was in) → {library, kind, item, group: name or null}",
+            "{library?, kind, item: name (graphic: id or name), group?: name (null or none: out of its group), before?: item | after?: item (an item of the same kind, named as `item` is)} put an item in a group (out of the one it was in); with `before` or `after`, also move it just before or after that item in the order its kind is shown in, as dragging it in the Libraries panel does → {library, kind, item, group: name or null}",
             always,
             move_item
         ),
+        cmd!(
+            "library.moveGroup",
+            "Move Library Group",
+            [],
+            None,
+            "{library?, group: name, to: index} move a group to position `to` of the library's groups (0 = first; past the end: last), as dragging its header in the Libraries panel does → {library, group, to: where it went}",
+            always,
+            move_group
+        ),
         cmd!(
             query "library.export",
             "Export Library",
@@ -1023,13 +1052,30 @@ fn move_item(s: &mut Session, p: &Value) -> Result<Value> {
     const C: &str = "library.moveItem";
     let (id, kind, item) = item_param(s, p, C)?;
     let group = str_param(p, "group");
+    let next_to = match (str_param(p, "before"), str_param(p, "after")) {
+        (Some(_), Some(_)) => return Err(bad(C, "give `before` or `after`, not both")),
+        (Some(b), None) => Some((b, false)),
+        (None, Some(a)) => Some((a, true)),
+        (None, None) => None,
+    };
     let (key, to) = s.libraries.change(&id, |lib| {
-        let r = lib.item_ref(kind.item_kind(), item).ok_or_else(|| bad(C, format!("the library has no item `{item}`")))?;
+        let missing = |item: &str| bad(C, format!("the library has no item `{item}`"));
+        let r = lib.item_ref(kind.item_kind(), item).ok_or_else(|| missing(item))?;
         let at = group.map(|g| group_at(lib, g, C)).transpose()?;
+        let next_to = next_to
+            .map(|(other, after)| Ok::<_, EngineError>((lib.item_ref(kind.item_kind(), other).ok_or_else(|| missing(other))?, after)))
+            .transpose()?;
+        if let Some((other, after)) = &next_to
+            && *other != r
+        {
+            lib.place_next_to(&r, other, *after);
+        }
         lib.ungroup(&r);
         let key = r.item.clone();
         let to = at.and_then(|i| lib.groups.get_mut(i)).map(|g| {
-            g.items.push(r);
+            // Next to the other item when that is in the group too, else last.
+            let near = next_to.as_ref().and_then(|(other, after)| g.items.iter().position(|x| x == other).map(|j| j + usize::from(*after)));
+            g.items.insert(near.unwrap_or(g.items.len()).min(g.items.len()), r);
             g.name.clone()
         });
         Ok((key, to))
@@ -1037,6 +1083,22 @@ fn move_item(s: &mut Session, p: &Value) -> Result<Value> {
     Ok(json!({ "library": id, "kind": str_param(p, "kind"), "item": key, "group": to }))
 }
 
+fn move_group(s: &mut Session, p: &Value) -> Result<Value> {
+    const C: &str = "library.moveGroup";
+    let id = library_id(s, p, C)?;
+    let group = group_param(p, C)?;
+    let to = p.get("to").and_then(Value::as_u64).ok_or_else(|| bad(C, "missing `to` (an index)"))?;
+    let (name, at) = s.libraries.change(&id, |lib| {
+        let i = group_at(lib, group, C)?;
+        let g = lib.groups.remove(i);
+        let at = usize::try_from(to).unwrap_or(usize::MAX).min(lib.groups.len());
+        let name = g.name.clone();
+        lib.groups.insert(at, g);
+        Ok((name, at))
+    })?;
+    Ok(json!({ "library": id, "group": name, "to": at }))
+}
+
 fn export(s: &mut Session, p: &Value) -> Result<Value> {
     const C: &str = "library.export";
     let id = library_id(s, p, C)?;