fix: close the review findings on the last web wave

Full verification of the XREF/data/ARCHIVE/worker wave (three
reviewers, one forensic) found no blockers and no crossfire damage;
five majors and several minors are fixed here.

- ARCHIVE now packages XREFs: the bytes registry added by the same
  commit is consulted (the in-code 'no xref store exists' comment was
  false within the commit), and the drawing is packaged only after
  the full save preparation — without it kernel solids shipped
  without their ACIS data.
- PendingClose stores the stable tab id instead of a vector index:
  the web save now completes asynchronously, and closing a lower tab
  during the serialization window shifted indices — closing the WRONG
  tab, or panicking on a stale one. All producers and consumers
  resolve the id at use.
- The xref bytes registry is capped (6 entries, oldest evicted): every
  open, drop, and pick registered full drawing bytes with no bound —
  an OOM waiting to happen. Reopened recents register too, so any
  entry path can serve as another drawing's xref.
- A re-picked data-link source with the same name but different bytes
  no longer silently replaces the first (the browser hands back bare
  names, so two same-named files collided and every reference read
  the newer pick): the pick is refused and the user is told why.
- The unresolved-xref warning lists file names (what to drop on the
  page), not internal block names; the ZIP writer refuses the ZIP64
  sentinel values (0xFFFF counts, 0xFFFFFFFF sizes/offsets) instead
  of emitting archives that send strict readers hunting for a ZIP64
  EOCD that is absent; docs note the worker-side saves.

Known follow-ups documented, not fixed here: the serde-skipped
ExtendedEntity/CadDocument raw bytes still drop on worker crossings
(needs acadrust accessors — upstream), and WBLOCK/SAVEALL/ARCHIVE
still serialize inline (the worker route is a small follow-up).

src/app/mod.rs

@@ -1298,8 +1298,10 @@ pub(super) struct OpenCADStudio {
 /// What triggered the "unsaved changes" dialog.
 #[derive(Debug, Clone)]
 pub(super) enum PendingClose {
-    /// User tried to close the tab at this index.
-    Tab(usize),
+    /// User tried to close this tab — stored as the stable tab id, because
+    /// the web save completes asynchronously and removing an earlier tab in
+    /// the meantime shifts every later vector index.
+    Tab(u64),
     /// User tried to quit the application.
     Quit,
 }