feat: implement -R/--relative, --no-implied-dirs, --dirs/-d, --mkpath

RSYNC_COMPAT Phase-2 row M: path-list construction and destination
directory creation while preserving traversal safety.

- -R/--relative with --files-from: transmit each listed entry under its
  bare relative destination path (no source-root mirror). Files keep an
  absolute local read path plus a separate wire/dest path (File.send_path);
  manifest, incremental quick-check and change output follow the wire path,
  so --delete and --remove-source-files stay consistent. -R without
  --files-from is unchanged (full mirror).
- --no-implied-dirs: client-only, only meaningful with -R + --files-from.
  A listed file whose parent dir is not itself (or via an ancestor)
  explicitly listed cannot be placed; the run fails up front with a clear
  error. No effect otherwise.
- --dirs/-d + --old-dirs/--old-d aliases: -d <dir> transmits the source
  root as an explicit empty directory entry (STATUS_MKDIR frame); with
  --files-from listed dirs are created empty and listed files transferred,
  never descending. Works single-threaded, -m (sequential scanner in the
  -m scan thread) and chunk-serialization (per-file type marker).
  Directory entries appear in the delete manifest.
- --mkpath: new wire bool; server creates the destination root (and missing
  leading components under its authorized root) at connection start. A
  missing destination root is now rejected by default.
- Protocol bumped to 2.7.0 (mkpath wire field + STATUS_MKDIR + chunk type
  marker). All receive paths funnel through file_save_to_disk_full which
  creates directories via the secure confined mkdir engine; dir entries are
  excluded from --remove-source-files outcome acknowledgements on both ends.
- Unit coverage: CLI parse (relative/dirs aliases/mkpath/no-implied-dirs),
  config round-trip (relative + mkpath), scanner --dirs non-recursion and
  -R send_path (sequential + parallel), receiver dir-entry save.
- Integration coverage: TestRelativeFilesFrom, TestNoImpliedDirs, TestDirs,
  TestMkpath (single and -m).
- RSYNC_COMPAT: 4 rows move to Implemented (Summary 67/3/5/1/71 = 147).

src/server/receiver.c

@@ -133,7 +133,8 @@ int receiver_process(Config* config, int file_descriptor, const ReceiverSink* si
   if (!receive_status(file_descriptor, &status))
     return -1;
   while (status == STATUS_NEXT || status == STATUS_CHUNK || status == STATUS_CHECK ||
-         status == STATUS_KEEPALIVE || status == STATUS_ABORT || status == STATUS_CHECK_BATCH) {
+         status == STATUS_KEEPALIVE || status == STATUS_ABORT || status == STATUS_CHECK_BATCH ||
+         status == STATUS_MKDIR) {
     if (status == STATUS_KEEPALIVE) {
       if (!send_status(file_descriptor, STATUS_KEEPALIVE))
         return -1;
@@ -156,6 +157,10 @@ int receiver_process(Config* config, int file_descriptor, const ReceiverSink* si
       if (!receiver_process_batch(config, file_descriptor))
         return -1;
       goto next;
+    } else if (status == STATUS_MKDIR) {
+      File* dir = file_receive_directory(file_descriptor);
+      if (!dir || !sink->store_file(dir, sink->context))
+        goto receive_error;
     } else {
       File* file = file_receive(config, file_descriptor);
       if (!file) {
@@ -208,7 +213,7 @@ static bool receiver_save_file(File* file, void* context_pointer) {
   } else {
     result = file_save_to_disk_full(context->config->receive_root_directory, file, context->config);
   }
-  if (result != FILE_SAVE_ERROR && context->config->remove_source_files &&
+  if (result != FILE_SAVE_ERROR && context->config->remove_source_files && !file->is_dir &&
       !receiver_outcomes_append(&context->outcomes, (unsigned char)result)) {
     file_destroy(file);
     return false;