web: XREFs, data links & extraction, ARCHIVE transmittals, worker-side saves

The last four big web-vs-desktop gaps.

XREFs: XATTACH reads the picked drawing's bytes and registers them
name-keyed (the image/PDF-underlay pattern), pre-parses on the web
worker, and the commit merge produces the same xref block flow as
native. Opened drawings resolve xrefs from the registry (trailing-name
match) before cache build; unresolved references produce ONE warning
listing the names and the fix — drop the referenced files on the page
(drops and picker opens now register their bytes) and reopen or
XRELOAD. XRELOAD works off the registry on unsaved drawings too.

Data Link & Extraction: spreadsheet links read picked xlsx/csv bytes
(calamine from slices / CSV from string) through a session registry,
so preview, refresh, and table-insert work on web; extraction takes
external drawings via a multi-file picker parsed on the worker, and
outputs CSV/TXT/.dxex via download_bytes. The disabled 'Add folder'
button explains why.

ARCHIVE/ETRANSMIT: assembles the drawing plus registered images, PDF
underlays, and session plot tables into a deterministic ZIP (minimal
writer over flate2 — no new dependency) delivered as
{stem}_transmit.zip; skipped dependencies are reported by name.

Save/autosave off the UI thread: the parse worker gains a serialize
action (protocol v5) — the document is prepared once on the main
thread (clone, styles, dimension bake), then the DWG/DXF writer pass,
the dominant cost, runs on the worker; any worker failure falls back
to inline serialization with the identical shared implementation.
The save path also clears the dirty flag only when the tab didn't
change mid-write, and the runtime-fields sidecar now carries
graphic_data, fixing a pre-existing loss of raw DWG round-trip bytes
on web-opened documents.

web/ocs-parse-worker.js

@@ -1,7 +1,7 @@
-import init, { parse_document, sha256_document } from "./worker_pkg/ocs_web_worker.js?v=4";
+import init, { parse_document, serialize_document, sha256_document } from "./worker_pkg/ocs_web_worker.js?v=5";
 
 const ready = init(
-  new URL("./worker_pkg/ocs_web_worker_bg.wasm?v=4", import.meta.url),
+  new URL("./worker_pkg/ocs_web_worker_bg.wasm?v=5", import.meta.url),
 );
 
 self.onmessage = async ({ data }) => {
@@ -13,6 +13,18 @@ self.onmessage = async ({ data }) => {
       self.postMessage({ ok: true, digest });
       return;
     }
+    if (data.action === "serialize") {
+      // SAVE/AUTOSAVE: decode the prepared document, run the DWG/DXF writer
+      // here, and hand the encoded result buffer back. Mirrors the parse
+      // branch's copy-before-transfer (wasm-bindgen returns a view into
+      // WebAssembly.Memory, which must not detach).
+      const encoded = serialize_document(new Uint8Array(data.document), (next) => {
+        stage = next;
+      });
+      const transferable = encoded.slice();
+      self.postMessage({ ok: true, data: transferable.buffer }, [transferable.buffer]);
+      return;
+    }
     const encoded = parse_document(
       data.name,
       new Uint8Array(data.bytes),