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.

src/shared/config.h

@@ -147,7 +147,7 @@ typedef struct Config {
   bool skip_compress_set;
 } Config;
 
-#define PROTOCOL_VERSION "2.4.0"
+#define PROTOCOL_VERSION "2.5.0"
 #define DEFAULT_CHUNK_SIZE (10 * 1024 * 1024)
 
 Config* config_create(void);