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/shared/file.c

@@ -73,6 +73,7 @@ File* file_create(const char* path) {
 
   memcpy(file->path, path, path_len);
   file->path[path_len] = '\0';
+  file->send_path = NULL;
   file->data = data_create_reserve(0);
   if (file->data == NULL) {
     free(file->path);
@@ -81,6 +82,7 @@ File* file_create(const char* path) {
   }
   file->metadata = NULL;
   file->skip = false;
+  file->is_dir = false;
   return file;
 }
 
@@ -94,6 +96,8 @@ void file_destroy(void* item) {
   file->metadata = NULL;
   free(file->path);
   file->path = NULL;
+  free(file->send_path);
+  file->send_path = NULL;
   free(file);
 }
 
@@ -323,6 +327,27 @@ bool file_ensure_directory_secure(const char* path) {
   return ok;
 }
 
+/* True when `path` resolves to an existing directory below the authorized root
+ * (never creating anything). Used by the server to decide whether a client's
+ * destination root already exists. */
+bool file_directory_exists_secure(const char* path) {
+  if (!path)
+    return false;
+  char* leaf = NULL;
+  int parent_fd = file_open_secure_parent(path, &leaf, false);
+  if (parent_fd < 0)
+    return false;
+  int dir_fd = openat(parent_fd, leaf, O_RDONLY | O_DIRECTORY | O_NOFOLLOW | O_CLOEXEC);
+  if (dir_fd < 0 && errno == ENOENT)
+    dir_fd = -1;
+  bool ok = dir_fd >= 0;
+  if (dir_fd >= 0)
+    close(dir_fd);
+  close(parent_fd);
+  free(leaf);
+  return ok;
+}
+
 bool file_rename_secure(const char* old_path, const char* new_path) {
   char *old_leaf = NULL, *new_leaf = NULL;
   int old_parent = file_open_secure_parent(old_path, &old_leaf, false);