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/server/receiver.h

@@ -3,16 +3,37 @@
 
 #include "config.h"
 #include "file.h"
+#include "file_receive.h"
 
 typedef bool (*ReceiverFileSink)(File* file, void* context);
 
+/* Ordered per-file save outcomes for one connection.  One entry is appended
+   for every data-bearing file the receiver processes (in the order the files
+   were sent) so the sender of a --remove-source-files transfer can be told
+   which sources were actually written versus skipped on the receiver. */
+typedef struct {
+  unsigned char* entries; /* FILE_SAVE_WRITTEN or FILE_SAVE_SKIPPED */
+  size_t count;
+  size_t capacity;
+} ReceiverOutcomes;
+
+typedef bool (*ReceiverSuccessFrame)(int fd, void* context);
+
 typedef struct {
   ReceiverFileSink store_file;
   void* context;
   bool send_error;
   bool send_success;
+  /* Emits the end-of-transfer success frame.  When the sender requested
+     --remove-source-files this includes one per-file status per processed
+     data file followed by the final STATUS_OK; otherwise just STATUS_OK. */
+  ReceiverSuccessFrame send_success_frame;
 } ReceiverSink;
 
+bool receiver_outcomes_append(ReceiverOutcomes* outcomes, unsigned char code);
+void receiver_outcomes_destroy(ReceiverOutcomes* outcomes);
+bool receiver_send_final_success(int fd, const Config* config, const ReceiverOutcomes* outcomes);
+
 int receiver_process(Config* config, int file_descriptor, const ReceiverSink* sink);
 int receiver_receive_files(Config* config, int file_descriptor);