Export Workspace Endpoint

Export a workspace as a .qualia archive (ZIP), streamed to the client.

Carries workspace data only. Workspace files are available exclusively through export-to-path: an archive with a workspace in it runs to gigabytes, and every route that moves those bytes through the renderer has crashed it, taking the app's socket down with it.

When some notebooks or images cannot be read the archive is still produced. X-Export-Incomplete carries the total skip count and the X-Export-Skipped-* headers break it down, with the skipped notebook paths and blob IDs recorded in workspace.json.