An upload through the File Explorer is now sent in 8 MB parts that the
server stages on the host (<data dir>/upload-staging/<user>/<id>/,
server/lib/upload-staging.js) and writes only once every byte is there,
through the same upload/uploadBatch code and guards (?staged=<id>). A
restart, an update or a crash pauses it: the browser waits up to 3 min,
asks how much arrived (GET /fs/upload-part) and sends the rest; the dock
says the dashboard is restarting meanwhile. A commit's answer is stored
and replayed, never written twice; a refused one (EEXISTS) keeps the
bytes for a retry with another conflict policy.
A file uploaded into a member's folder is now written to a hidden temp
name and moved onto its name only when whole: write-as cannot tell a
dropped connection from the end of a file, and `replace` left a truncated
file under the real name when the server died mid-upload.
New: GET|POST|DELETE /api/{lanes/:id,homes/:who}/fs/upload-part; 409
EOFFSET/EINCOMPLETE/EBUSY, 429 ETOOMANY. Verified on a sandbox server:
SIGTERM mid-upload of a 60 MB file in a real browser, resumed and
sha256-identical.
