fix(io): resolve root named-objects dict robustly on foreign DWGs

Some DWGs written by other programs leave the root named-objects
dictionary unrecoverable: the header pointer names a non-dictionary and
the real NOD child dicts are owned by a table-control handle, so nothing
has owner==NULL. Navigating the root then silently no-ops, and
registering a new named-object entry (page setup, annotation scale, CTAB)
against the missing root orphans it — it never round-trips.

Add shared annotative::root_named_dict_handle(): resolve the header
pointer, else adopt the richest owner==NULL dict, else synthesise a fresh
root and write it back. Route ensure_plotsettings_dict, add_scale's
scalelist-create branch, and set_saved_active_layout through it. Resolve
the plot-settings dict via the PlotSettings owner when the header pointer
is stale, mirroring scalelist_dict_handle.

Verified page setups survive a DWG save/reload on the repro file with
layouts intact.

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

src/scene/mod.rs

@@ -1967,7 +1967,11 @@ impl Scene {
         let scalelist_h = match self.scalelist_dict_handle() {
             Some(h) => h,
             None => {
-                let root_h = self.document.header.named_objects_dict_handle;
+                // Resolve (or synthesise) the root robustly — a drawing whose
+                // header root pointer is unresolvable would otherwise register
+                // the new dictionary against a missing root and silently orphan
+                // it. See `annotative::root_named_dict_handle`.
+                let root_h = crate::scene::annotative::root_named_dict_handle(&mut self.document);
                 let h = self.document.allocate_handle();
                 let mut d = Dictionary::new();
                 d.handle = h;