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.

src/app/mod.rs

@@ -2305,6 +2305,22 @@ pub enum Message {
         bounds: Option<iced::Rectangle>,
         screenshot: Option<iced::window::Screenshot>,
     },
+    /// Web SAVE: the serializer worker (or the inline fallback) produced the
+    /// drawing bytes; download them and finish the save bookkeeping. The
+    /// epoch/revision/camera markers snapshot the tab when serialization
+    /// started — the web counterpart of native's `snapshot_is_current`, so a
+    /// save that raced user edits does not wipe the newer dirty flag.
+    #[cfg(target_arch = "wasm32")]
+    WebSaveSerialized {
+        tab_id: u64,
+        filename: String,
+        autosave_key: String,
+        version: acadrust::DxfVersion,
+        geometry_epoch: u64,
+        edit_revision: u64,
+        camera_generation: u64,
+        result: Result<Vec<u8>, String>,
+    },
     /// Retry the failed save after the other application releases the file.
     #[cfg(not(target_arch = "wasm32"))]
     SaveFileInUseRetry,
@@ -3579,7 +3595,9 @@ pub enum Message {
     DataExtractionBrowseSettings,
     DataExtractionBrowseSettingsResult(Option<std::path::PathBuf>),
     DataExtractionAddDrawings,
-    DataExtractionAddDrawingsResult(Vec<std::path::PathBuf>),
+    /// Picked source drawings — `Err` carries a pick/parse failure (web
+    /// parses each file on the worker at pick time).
+    DataExtractionAddDrawingsResult(Result<Vec<std::path::PathBuf>, String>),
     DataExtractionAddFolder,
     DataExtractionAddFolderResult(Option<std::path::PathBuf>),
     DataExtractionClearSources,