refactor: split four oversized files into submodule trees

Break up the four giant files (~29k lines total) into focused submodule
directories, one concern per file. Pure mechanical moves — behavior is
unchanged.

- app/update.rs (9650) -> app/update/{mod,viewport,command,style,file,
  dynamic,dialog,util}. The 384-arm update_inner match stays in mod.rs
  with thin delegating arms; the 66 fattest arm bodies move to topic
  methods. Arm set preserved exactly (384 -> 384).
- scene/mod.rs (9017) -> scene/{mod,entity,tess,hittest,layout,paper,
  mspace,project,selection,modify,group_layer,preview}. The impl Scene
  body is split across files via inherent-impl-per-file; struct, ctor,
  fields and load-time helpers stay in the root. fn count 123 -> 123.
- app/commands.rs (6082) -> app/commands/* grouped by command family.
  The 233-arm dispatch match becomes source-ordered per-family
  dispatch_* handlers, preserving first-match precedence (233 -> 233).
- app/view.rs (4686) -> app/view/{mod,overlay,modal,viewcube,controls}.

Largest file drops from 9650 to 3343. Builds clean; app launches and
loads plugins with no panic.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/scene/group_layer.rs

@@ -0,0 +1,98 @@
+// Auto-split from scene/mod.rs. Pure text-move; behaviour unchanged.
+use super::*;
+
+impl Scene {
+    // ── Group helpers ──────────────────────────────────────────────────────
+
+    pub fn groups(&self) -> impl Iterator<Item = &acadrust::objects::Group> {
+        self.document.objects.values().filter_map(|obj| match obj {
+            ObjectType::Group(g) => Some(g),
+            _ => None,
+        })
+    }
+
+    /// Returns the names of all groups that contain `handle`.
+    pub fn group_names_for_entity(&self, handle: Handle) -> Vec<String> {
+        self.groups()
+            .filter(|g| g.contains(handle))
+            .map(|g| g.name.clone())
+            .collect()
+    }
+
+    /// Creates a named group from the given handles and registers it in the group dictionary.
+    pub fn create_group(&mut self, name: String, handles: Vec<Handle>) -> Handle {
+        let group_dict_handle = self.document.header.acad_group_dict_handle;
+        let mut group = acadrust::objects::Group::new(&name);
+        group.handle = self.document.allocate_handle();
+        group.owner = group_dict_handle;
+        group.add_entities(handles);
+        let gh = group.handle;
+        self.document.objects.insert(gh, ObjectType::Group(group));
+        if let Some(ObjectType::Dictionary(dict)) =
+            self.document.objects.get_mut(&group_dict_handle)
+        {
+            dict.add_entry(&name, gh);
+        }
+        gh
+    }
+
+    /// Dissolves all groups that contain any of the given handles.
+    /// Returns the number of groups removed.
+    pub fn delete_groups_containing(&mut self, handles: &[Handle]) -> usize {
+        let group_dict_handle = self.document.header.acad_group_dict_handle;
+        let to_delete: Vec<Handle> = self
+            .document
+            .objects
+            .values()
+            .filter_map(|obj| match obj {
+                ObjectType::Group(g) if handles.iter().any(|h| g.contains(*h)) => Some(g.handle),
+                _ => None,
+            })
+            .collect();
+        let count = to_delete.len();
+        for gh in &to_delete {
+            if let Some(ObjectType::Dictionary(dict)) =
+                self.document.objects.get_mut(&group_dict_handle)
+            {
+                dict.entries.retain(|(_, h)| h != gh);
+            }
+            self.document.objects.remove(gh);
+        }
+        count
+    }
+
+    /// If `handle` belongs to any selectable groups, also select all other members of those groups.
+    pub fn expand_selection_for_groups(&mut self, handles: &[Handle]) {
+        let to_add: Vec<Handle> = self
+            .document
+            .objects
+            .values()
+            .filter_map(|obj| match obj {
+                ObjectType::Group(g) if g.selectable && handles.iter().any(|h| g.contains(*h)) => {
+                    Some(g.entities.clone())
+                }
+                _ => None,
+            })
+            .flatten()
+            .collect();
+        for h in to_add {
+            self.selected.insert(h);
+        }
+        self.bump_selection();
+    }
+
+    // ── Layer helpers ──────────────────────────────────────────────────────
+
+    pub fn toggle_layer_visibility(&mut self, name: &str) {
+        if let Some(layer) = self.document.layers.get_mut(name) {
+            layer.flags.off = !layer.flags.off;
+        }
+        self.bump_geometry();
+    }
+
+    pub fn toggle_layer_lock(&mut self, name: &str) {
+        if let Some(layer) = self.document.layers.get_mut(name) {
+            layer.flags.locked = !layer.flags.locked;
+        }
+    }
+}