Viewport block caches no longer scan every object per entity

A drawing with 650k entities and 21k objects stalled for 7 minutes the
first time a layout viewport was drawn (#1746): building that viewport's
block cache asked for each entity's layer viewport overrides, and the
codec found each layer's extension dictionary by scanning every object
whenever the layer had none. Codec 2646725 reads the registered link.

The XCLIP check made the same scan for every nested INSERT without an
extension dictionary; it now reads the INSERT's own link, which the
readers and ensure_extension_dictionary always set.

Cargo: codec 2646725, kernel 4e6334a, graph 2fd1d42.

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

src/scene/pick/xclip.rs

@@ -48,15 +48,12 @@ fn resolve_extension_dictionary(doc: &CadDocument, owner: Handle) -> Option<Hand
         })
 }
 
-/// The INSERT's extension dictionary through its own link or the document's
-/// index — both O(1). Render walks ask this for every INSERT, so it must not
-/// fall back to [`resolve_extension_dictionary`]'s scan of every object.
-fn linked_extension_dictionary(doc: &CadDocument, ins: &Insert) -> Option<Handle> {
-    ins.common
-        .xdictionary_handle
-        .filter(|h| !h.is_null())
-        .or_else(|| doc.extension_dictionary_handle(ins.common.handle))
-        .filter(|h| !h.is_null())
+/// The INSERT's extension dictionary through its own link. Render walks ask
+/// this for every INSERT, so it must not fall back to a scan of every object:
+/// [`CadDocument::extension_dictionary_handle`] scans for an entity without
+/// a linked dictionary. Readers and `ensure_extension_dictionary` set the link.
+fn linked_extension_dictionary(ins: &Insert) -> Option<Handle> {
+    ins.common.xdictionary_handle.filter(|h| !h.is_null())
 }
 
 /// Resolve the enabled XCLIP spatial filter for `ins`, if any.
@@ -66,7 +63,7 @@ fn linked_extension_dictionary(doc: &CadDocument, ins: &Insert) -> Option<Handle
 /// filter, it is disabled (code 71 = 0), or it has fewer than two boundary
 /// points.
 pub fn insert_spatial_filter<'a>(doc: &'a CadDocument, ins: &Insert) -> Option<&'a SpatialFilter> {
-    let xdict = linked_extension_dictionary(doc, ins)?;
+    let xdict = linked_extension_dictionary(ins)?;
     let acad_filter = dict_entry(doc, xdict, "ACAD_FILTER")?;
     let spatial = dict_entry(doc, acad_filter, "SPATIAL")?;
     match doc.objects.get(&spatial)? {
@@ -84,7 +81,7 @@ pub fn filter_handle(doc: &CadDocument, insert: Handle) -> Option<Handle> {
     let Some(codec::EntityType::Insert(ins)) = doc.get_entity(insert) else {
         return None;
     };
-    let xdict = linked_extension_dictionary(doc, ins)?;
+    let xdict = linked_extension_dictionary(ins)?;
     let acad_filter = dict_entry(doc, xdict, "ACAD_FILTER")?;
     let spatial = dict_entry(doc, acad_filter, "SPATIAL")?;
     matches!(