rsync's %c counts the block-checksum bytes received: even a whole-file
transfer with no basis receives rsync's 16-byte sum header (append and
inplace included), while a dry run receives nothing. FastSync's
whole-file path has no equivalent header, so report 16 for parity when
delta is inactive, keep 0 for dry runs, and keep the real received bytes
when delta is active (FastSync's signature framing differs, so delta %c
stays divergent). Add a strict %c/%l/%n differential against rsync and
turn the %b check into a real rsync differential (semantics: both count
wire bytes and exceed %l; the exact values are protocol-specific).
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