Add --log/--remote-log diagnostic tracing for stalled transfers

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.

src/main.rs

@@ -8,6 +8,7 @@ mod protocol;
 mod server;
 mod ssh;
 mod tail;
+mod tracelog;
 mod zstd_frame;
 
 use clap::Parser;