Adds an optional timestamped trace of transfer-relevant events (tail
position vs. file length, rename detection, finalize, protocol
send/recv) to both the client and server sides, so a stalled transfer
can be diagnosed from correlated local/remote logs instead of guessing.
Notably logs when a source's final size ends up smaller than what was
already read (pos > current_len after rename) -- this can never
satisfy the pos == current_len finalize condition, which is a real
deadlock risk for writers that preallocate a fixed-size file and
truncate down to the actual capture length before renaming.
--log <path> on either the client or `--server` process traces that
process's own events. --log on ClientCli is local-only; --remote-log
<path> is forwarded as the spawned --server process's --log so both
sides of a remote transfer leave a trace, tied together by timestamp.
ServerCli has no --extension field -- run_send_server already gets
fully-resolved --partial and --final-source from the client, so the
extension is never needed remotely. Passing it made every remote-source
pull fail with "unexpected argument '--extension' found".
scp's "host:" (no path after the colon) means the remote home
directory, reusing the source's basename. resolve_dest_paths only
recognized a directory target via a trailing '/', so an empty path
fell through and was used literally as the filename, producing an
empty --dest-final and hanging the remote --server --recv.
eric