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.

Cargo.toml

@@ -8,6 +8,7 @@ default-run = "OpenCADStudio"
 [workspace]
 members = [
     "crates/ocs_plugin_api",
+    "crates/ocs_save",
     "crates/ocs_web_worker",
     "crates/dwg-thumbnailer",
     "crates/dwg-thumbnailer-win",
@@ -36,6 +37,9 @@ acadrust = { git = "https://github.com/HakanSeven12/cadcodec.git", rev = "568a12
 cadkernel = { git = "https://github.com/HakanSeven12/cadkernel.git", rev = "dcf0840", features = ["acis", "offset"] }
 cadkernel-constraints = { git = "https://github.com/HakanSeven12/cadkernel.git", rev = "dcf0840" }
 dwg-thumbnailer = { path = "crates/dwg-thumbnailer" }
+# Shared SAVE serialization dispatch + web-worker wire format: one
+# implementation for the inline (fallback/native) path and the worker path.
+ocs_save = { path = "crates/ocs_save" }
 flate2 = "1"
 image = { version = "0.25", default-features = false, features = ["png", "jpeg", "bmp", "tiff"] }
 hayro = "0.7.1"