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).

tests/test_file.c

@@ -886,6 +886,42 @@ static void test_inplace_overwrite_truncates_shorter_payload() {
   rmdir(root);
 }
 
+/* Explicit directory entries (--dirs) create the directory under the receive
+   root through the same save funnel, creating parents as needed, and reject
+   traversal the same way a file path does. */
+static void test_dir_entry_save_to_disk() {
+  const char* root = "test_dir_entry_root";
+  EXPECT_EQ_INT(mkdir(root, 0755), 0);
+
+  Config* config = config_create();
+  EXPECT_NOT_NULL(config);
+
+  File* dir = file_create("alpha/beta/gamma");
+  EXPECT_NOT_NULL(dir);
+  dir->is_dir = true;
+  EXPECT_EQ_INT(file_save_to_disk_full(root, dir, config), FILE_SAVE_WRITTEN);
+  EXPECT_EQ_INT(file_save_to_disk_full(root, dir, config), FILE_SAVE_WRITTEN);
+  file_destroy(dir);
+
+  struct stat st;
+  EXPECT_EQ_INT(stat("test_dir_entry_root/alpha/beta/gamma", &st), 0);
+  EXPECT_TRUE(S_ISDIR(st.st_mode));
+
+  /* The directory-entry save path never follows or escapes. */
+  File* evil = file_create("../dir_entry_escape");
+  EXPECT_NOT_NULL(evil);
+  evil->is_dir = true;
+  EXPECT_EQ_INT(file_save_to_disk_full(root, evil, config), FILE_SAVE_ERROR);
+  file_destroy(evil);
+  EXPECT_EQ_INT(lstat("../dir_entry_escape", &st), -1);
+
+  config_delete(config);
+  rmdir("test_dir_entry_root/alpha/beta/gamma");
+  rmdir("test_dir_entry_root/alpha/beta");
+  rmdir("test_dir_entry_root/alpha");
+  rmdir(root);
+}
+
 void test_file() {
   test_file_create();
   test_file_destroy_null();
@@ -906,6 +942,7 @@ void test_file() {
   test_file_content_to_buffer();
   test_file_save_to_disk_path_traversal();
   test_file_save_to_disk_deep_traversal();
+  test_dir_entry_save_to_disk();
   if (!is_running_under_valgrind()) {
     // Fork tests are skipped under valgrind because the parent process runs
     // orders of magnitude slower than the child (parent is instrumented, child