Package: File › Package collects and writes the package on the background thread

File › Package… read every linked file and font and wrote the package
on the UI thread, so the window stopped responding until the package
was written. For a document that links 50 images of 21 MB each, OK
with Background Export off, which does the same work, takes a median
of 2.25 s in a release build on Windows.

With Enable Background Export on (Preferences › File Handling), OK now
takes the options, the report's Document Info text and a snapshot of
the document, and closes the dialog. The thread that writes background
saves and exports then collects the links and fonts and writes the
package. The same OK takes 0.001 s, and the package holds the same
bytes. A save of unsaved changes that Package starts is written first.
With the preference off, on the web and over MCP and scripts, the
package is made at once with the same reply as before.

Docs: docs/target-app-parity.md.

docs/target-app-parity.md

@@ -865,6 +865,7 @@ Moved from the ROADMAP's "Shipped so far" on 2026-10-10. Update it when a featur
   - File › Place reads Word `.docx` documents as area type with the same formatting, taken from the document defaults, the paragraph and character styles (their toggle properties reversing each other) and direct formatting, the theme's fonts included. Table rows are paragraphs with their cells separated by tabs; hidden and deleted text, field instructions, pictures, headers, footers and notes are left out (`file.place`, `crates/engine/src/tests_textimport.rs`).
   - A Word paragraph justified with a low, medium or high kashida length places justified with Insert Kashidas set to Short, Medium or Long (`file.place`, `crates/engine/src/tests_textimport.rs`).
   - On the desktop, Copy and Cut make the system clipboard's formats (text, SVG, PDF, PNG) from a copy of the clipboard on the thread that writes background saves and exports, and write them when they are ready; while that thread has a save, an export, a print or a recovery copy to finish, they are made and written at once. Until they are written, Paste in VectorCraft takes the internal clipboard, and while a text field or the Type tool has the keyboard the app waits for them. Another Copy or Cut, or a copy of text in VectorCraft, drops them.
+  - On the desktop with Enable Background Export on (Preferences › File Handling), File › Package… collects the linked files and fonts and writes the package from the document as it was at OK on the thread that writes background saves and exports, with "Packaging" in the status bar and the busy spinner until it is written; a save of unsaved changes that Package starts is written first. `file.package` over MCP and scripts replies as before.
   - Missing linked files can be looked for in a folder and its subfolders: Find in Folder… in the Linked File Not Found dialog and in the Links panel menu lists the files with each one's name, those with the size and contents saved with the link first, and relinks the files chosen, copying an image's file into the document's folder when asked, without replacing a file there (`links.findFiles`, `links.relink {copy}`). In the Links panel it looks for the selected linked files when any are selected.
   - On Windows, a document opened from a mapped network drive finds its links stored with the share's own path (`\\server\share\…`), as the same document opened from the share does; links on other computers stay missing.
   - On Windows, a link on a drive mapped to a network share other than the document's (`Y:\…`) is missing, with the reason that the path is on another computer, as a link stored with that share's own path is; links on this computer's drives and on the document's share are followed.