feat: add structured per-file change output model (-i, --list-only, --out-format, --log-file-format)

Implement rsync-style itemized output backed by one shared change-event
engine (src/client/change_list.c):
- -i/--itemize-changes prints ">f+++++++++ <path>" for files actually sent
  (single-threaded and -m); unchanged files print nothing.
- --list-only prints an ls-style listing of files that would be transferred
  without contacting the server or writing anything.
- --out-format=FORMAT prints a printf-style template per changed file
  (tokens %%f %%n %%l %%b %%M %%%%; unknown escapes preserved).
- --log-file-format=FMT logs each transferred file when --log-file is set.
Events are emitted from the per-file sender path shared by both transfer
modes, so the single sender thread is the only reporter (no races).

tests/runner.c

@@ -1,5 +1,6 @@
 #include "test_array_list.h"
 #include "test_chunk.h"
+#include "test_change_list.h"
 #include "test_client_cli.h"
 #include "test_compression.h"
 #include "test_config.h"
@@ -40,6 +41,7 @@ int main() {
   RUN_TEST(test_array_list);
   RUN_TEST(test_shared_utils);
   RUN_TEST(test_chunk);
+  RUN_TEST(test_change_list);
   RUN_TEST(test_config);
   RUN_TEST(test_compression);
   RUN_TEST(test_scanner);