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/protocol.h

@@ -9,10 +9,16 @@
 /* Maximum allowed string size for receive_str (64 KB) */
 #define MAX_STRING_SIZE (64 * 1024)
 
-/* Maximum allowed data payload size for receive_data (100 MB) */
-#define MAX_DATA_PAYLOAD_SIZE (100ULL * 1024 * 1024)
-/* Maximum uncompressed file payload accepted by the receiver. */
-#define MAX_RECEIVE_FILE_SIZE (64ULL * 1024 * 1024)
+/* Maximum uncompressed file payload accepted by the receiver's whole-file
+ * paths.  A single whole file is charged against the per-connection memory
+ * reservation (MAX_CONNECTION_MEMORY) and against the server allocation
+ * ceiling (MAX_SERVER_ALLOC), so this mirrors those 256 MB bounds rather than
+ * the older 64 MB chunk-era cap.  Chunk-serialized payloads keep their own
+ * 64 MB cap (MAX_CHUNK_SIZE). */
+#define MAX_RECEIVE_WHOLE_FILE_SIZE (256ULL * 1024 * 1024)
+
+/* Maximum allowed data payload size for receive_data (whole-file bound) */
+#define MAX_DATA_PAYLOAD_SIZE MAX_RECEIVE_WHOLE_FILE_SIZE
 
 /* Maximum chunk size (64 MB) — prevents unbounded allocation from the wire */
 #define MAX_CHUNK_SIZE (64ULL * 1024 * 1024)