Upstream's 43e67d872f (feat: local models) reintroduced the old bespoke
ranged-parallel machinery (download_file/_probe_range_support/
_DOWNLOAD_CONNECTIONS) into the local-models router and deleted the
pm.downloader-based _download_job plus the pause/resume routes.
Re-swap all 3 call sites (model download, runtime-install leg, browsed
download) onto pm.downloader.Download via _download_job: one resumable
8-way parallel job per plan, progress via the shared tick callback,
partials in the managed partials root. Restore /download/pause +
/download/resume routes with the _RUNNING handle registry (dl handle
kept off the JSON job dict; resume re-runs the job body).
Restore the pre-clobber route tests (FakeRangeOpener stands in for
pm.downloader._OPENER with honest Range support) incl. the pause /
resume / finished-job-releases-handles coverage.
d967e9ed2b