fix: address c-review for -R/--dirs/--mkpath row

B1: destination-root existence/creation mishandled two legitimate forms.
file_directory_exists_secure/file_ensure_directory_secure now normalize a
trailing-slash destination (so the last component is never an empty leaf)
and treat a destination equal to the already-open authorized root as present
(no spurious <root>/<basename> nested dir with --mkpath). Confinement and
O_NOFOLLOW probing are unchanged. Regression integration tests: existing
dest with trailing slash works with and without --mkpath; dest == authorized
root works and creates no stray nested dir.

W1: chunk serialize/deserialize round-trip unit test for a directory entry
mixed with a regular file, with and without metadata; --dirs coverage under
-s and -s -m.
W2: --list-only and --dry-run now print file_wire_path() so all outputs show
the -R transformed relative name, matching -i/--out-format.
W3: RSYNC_COMPAT -d/--dirs note: dirs are created immediately under
--delay-updates (only regular files are staged).
W4: --dirs generator flushes chunks by element count too, so a long
--files-from list of empty directories cannot exceed the per-chunk file cap.
N1: STATUS_MKDIR added to status_to_string.
N2: removed dead TestMkpath._transfer.
N3: removed redundant --no-implied-dirs OPTION_TABLE row.
N4: integration tests: --dirs --delete keeps the just-created empty dir (and
deletes extras); a listed dir colliding with a regular file at the dest fails
cleanly.

RSYNC_COMPAT.md

@@ -74,7 +74,7 @@ This document maps rsync's full feature set to FastSync's current implementation
 | `-r`, `--recursive` | Recurse into directories | ✅ Implemented | Default behavior |
 | `-R`, `--relative` | Use relative path names | ✅ Implemented | Meaningful together with `--files-from` (FastSync's default full-tree scan always mirrors the full source argument path below the destination root, so -R does not change it). With `-R` + `--files-from` each listed entry is transmitted under its bare relative destination path: an entry `sub/x.txt` lands at `<dest>/sub/x.txt` (its leading components preserved) instead of under the `<dest>/<full source path>` mirror. Only the path sent on the wire changes; the client still reads the absolute source path, and the delete manifest derives from the sent (relative) paths so `--delete` and `--remove-source-files` stay consistent in both layouts. Works single-threaded and under `-m` (including chunk serialization) |
 | `--no-implied-dirs` | Don't send implied dirs with -R | ✅ Implemented | Client-side, meaningful only with `-R` + `--files-from`. rsync would normally create the ancestor directories implied by a listed file so it can be written; with `--no-implied-dirs` a listed file whose parent directory is not itself (or via an ancestor) explicitly listed cannot be placed, and FastSync fails the whole run up front with a clear error (`--no-implied-dirs: cannot place file '...': parent directory '...' is not explicitly listed`). Listing the directory (or an ancestor of it, or the whole tree `.`) permits the file. In every other mode the option has no effect. FastSync has no per-entry skip channel, so the rsync "omit the file" case is surfaced as a hard pre-transfer error |
-| `-d`, `--dirs`, `--old-dirs`, `--old-d` | Transfer dirs without recursing | ✅ Implemented | `-d <dir>` transmits an explicit directory entry for the source-root directory, so the destination mirror is created empty and nothing is descended into. With `--files-from` exactly the listed items are transferred: a listed directory is created empty (no descent) and a listed file is transferred with its content; the dest layout follows the same -R rules as plain files. A new wire frame (`STATUS_MKDIR`) carries each directory entry (path only); the receiver creates it with the same confined mkdir-parent semantics as regular writes, in single-threaded and `-m` receivers (chunk serialization carries a per-entry type marker). Directory entries appear in the delete manifest so `--delete` prunes correctly. FastSync divergences: directory mtimes/modes are not transmitted, filter/`--exclude` rules are not re-applied to the listed dirs mode (there is no descent during which they would apply), and `-d` never creates the intermediate directories between the destination root and a listed file beyond the usual on-demand parent creation |
+| `-d`, `--dirs`, `--old-dirs`, `--old-d` | Transfer dirs without recursing | ✅ Implemented | `-d <dir>` transmits an explicit directory entry for the source-root directory, so the destination mirror is created empty and nothing is descended into. With `--files-from` exactly the listed items are transferred: a listed directory is created empty (no descent) and a listed file is transferred with its content; the dest layout follows the same -R rules as plain files. A new wire frame (`STATUS_MKDIR`) carries each directory entry (path only); the receiver creates it with the same confined mkdir-parent semantics as regular writes, in single-threaded and `-m` receivers (chunk serialization carries a per-entry type marker). Directory entries appear in the delete manifest so `--delete` prunes correctly. FastSync divergences: directory mtimes/modes are not transmitted, filter/`--exclude` rules are not re-applied to the listed dirs mode (there is no descent during which they would apply), and `-d` never creates the intermediate directories between the destination root and a listed file beyond the usual on-demand parent creation. Under `--delay-updates` only regular files are staged: directory entries are created immediately, so a delayed run that fails part way can leave the already-created empty directories behind (matching rsync, which also creates directories as it processes the file list and only delays regular-file data) |
 | `--mkpath` | Create missing path components | ✅ Implemented | Wire option (client → server). At connection start the server creates the client's destination root directory (and any missing leading components below its own authorized root) when `--mkpath` is set, failing the connection cleanly if it cannot. Without `--mkpath` a destination root that does not exist yet is rejected up front (rsync semantics), so the flag is the only way to transfer into a not-yet-created destination directory. Creation is confined by the same secure mkdir walk as file writes (`O_NOFOLLOW`, no `..`) |
 
 ## 5. Transfer Modifications