FileSyncManager now accepts an optional bulk_upload_fn callback.
When provided, all changed files are uploaded in one call instead
of iterating one-by-one with individual HTTP POSTs.
DaytonaEnvironment wires this to sandbox.fs.upload_files() which
batches everything into a single multipart POST — ~580 files goes
from ~5 min to <2s on init.
Parent directories are pre-created in one mkdir -p call.
Fixes#7362 (item 1).
bff64858f9