beaconBackfiller.resume() already returns early when partialSyncComplete
is set, so in normal CL-driven operation the downloader never reaches
synchronise after the initial partial-state sync finishes. Add the same
guard at the synchronise entry point as defense in depth: any future
caller of synchronise (tests, other wiring) inherits the invariant
that partial-state nodes do not run full downloader cycles after
initial sync, even if the resume path is bypassed.
The check is cheap (one atomic.Load) and sits on the cold path, so the
impact on normal full-sync users is nil.
be19e2c67e