fix: #251 #252 #253 #255 #256 #257 receiver & remove-source correctness

#251 --remove-source-files deletes sources that were skipped receiver-side
     (--existing/--ignore-existing/--update). The receiver now reports a
     per-file outcome for every processed data file when the sender requests
     removal; the client only unlinks sources the receiver actually wrote.
     add remove_source_files to the wire config and bump the protocol to 2.5.0.
#252 --backup/--suffix/--backup-dir broken by NULL-vs-empty wire loss. Receivers
     canonicalize the empty wire string back to NULL for backup_dir, temp_dir,
     partial_dir and suffix, and --suffix is received unconditionally.
#253 --partial --partial-dir never installed completed files. file_save_to_disk
     now renames a fully written partial-dir file into the real destination.
#255 STATUS_CHECK read the entire old file before the size/mtime quick check.
     Old contents are only read when a checksum compare or delta needs them.
#256 receive_delta_file failure paths did not set *failed, so the caller sent
     STATUS_NEXT and waited for a body that never came. Every NULL return now
     marks the transfer failed.
#257 files >64 MiB could not transfer. Whole-file receive caps raised to the
     256 MiB connection/allocation ceiling (chunk caps stay 64 MiB) and the
     client ignores SIGPIPE so a server-side close surfaces as a clean error.

Unit tests added: config NULL-vs-empty round trip, incremental quick-check
skip/NEXT paths, delta oversize failure, partial-dir install, save-result
skip reporting.

README.md

@@ -484,10 +484,11 @@ defaults to the current directory. |
 
 ## Protocol and Security
 
-FastSync protocol version `2.4.0` is shared by the client and server. The
+FastSync protocol version `2.5.0` is shared by the client and server. The
 current protocol is sender-driven and includes configuration negotiation,
 including the maximum allocation limit, incremental checks, checksums,
-manifests, keep-alives, abort handling, and FastSync-native delta messages.
+manifests, keep-alives, abort handling, per-file remove-source results, and
+FastSync-native delta messages.
 Client and server versions must currently match exactly.
 
 TLS provides encrypted TCP transport. Supplying `--ca` enables certificate