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 refused 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 codec accessors — upstream), and WBLOCK/SAVEALL/ARCHIVE
still serialize inline (the worker route is a small follow-up).

(cherry picked from commit 6f429f24)

Ported onto main (#1717 review): main already keys PendingClose by tab
id, so those hunks stay as on main; the ZIP64 guard is moot with the zip
crate; the unresolved-xref names split on both path separators, since
Path does not split Windows paths in the browser build.

docs/native-vs-web.md

@@ -15,7 +15,7 @@ browser can't provide. This page lists the differences.
 | Fonts | Embedded stroke fonts + system TrueType + shaping + fallback | Fetched Noto subsets (shaping, fallback, and family aliases) + embedded stroke fonts |
 | File open | Native file dialog → path | Browser picker → bytes |
 | File save | Path / Save dialog → write to disk | Save dialog → direct browser download |
-| Parallelism | Multi-threaded (rayon) | Main thread + one Web Worker (DWG/DXF parse, hashing) |
+| Parallelism | Multi-threaded (rayon) | Main thread + one Web Worker (DWG/DXF parse, hashing, save/autosave serialization) |
 | GPU backend | Vulkan / DX12 / Metal (wgpu) | WebGL2 (the backend is pinned to OpenGL) |
 | Update check / external links | `ureq`, `open` | skipped / `window.open` |
 | External plugins | Native packages + Plugin Manager | **No** — native libraries cannot run in a browser |