fix: restore additional=["size","time"] to list_dir, update SPEC + tests

Root cause of DSM 408 was wrong path format (volume path vs share path),
not the additional parameter. Confirmed via test_additional.py that
json.dumps(["size","time"]) is the correct format; comma-separated string
is silently ignored by DSM.

- list_dir: restore 4-column table (Name, Type, Size, Modified)
- list_dir: use additional=json.dumps(["size","time"]) (confirmed working)
- SPEC.md: document share path requirement, additional format rules,
  note SYNO.FileStation.Stat unavailability, remove comma-sep gotcha
- tests: restore size/mtime mock data and assertions
- delete test_additional.py (throwaway diagnostic script)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

SPEC.md

@@ -84,9 +84,12 @@ List all shared folders visible to the authenticated user.
 
 **DSM call:** `SYNO.FileStation.List::list_share`
 ```
-additional=["real_path","volume_status"]
+additional=["volume_status"]
 ```
 
+> **Note:** DSM returns share paths directly (e.g. `/dev`, `/data`). The `path` field from the
+> response is used as-is — do not prepend the volume prefix.
+
 ---
 
 #### `list_dir`
@@ -95,7 +98,7 @@ List contents of a directory with optional pagination and sorting.
 **Parameters:**
 | Name | Type | Required | Default | Description |
 |------|------|----------|---------|-------------|
-| `path` | str | yes | — | Absolute path on the NAS |
+| `path` | str | yes | — | Share-relative path (e.g. `/dev`, `/data/photos`) |
 | `offset` | int | no | 0 | Number of items to skip |
 | `limit` | int | no | 100 | Max items to return (max 500) |
 | `sort_by` | str | no | `name` | `name`, `size`, `user`, `group`, `mtime`, `atime`, `crtime`, `posix`, `type` |
@@ -108,9 +111,21 @@ pagination context.
 ```
 folder_path={path}, offset={offset}, limit={limit},
 sort_by={sort_by}, sort_direction={sort_direction},
-additional=["real_path","size","time","perm","type"]
+additional=["size","time"]
 ```
 
+> **Path requirement:** `folder_path` must be a share path as returned by `list_share`
+> (e.g. `/dev`, `/data`). Volume paths (`/volume1/dev`) are **not accepted** and cause DSM
+> error 408.
+>
+> **`additional` format:** Must be a JSON array serialised as a string
+> (`json.dumps(["size","time"])` → `'["size", "time"]'`). A comma-separated string
+> (`"size,time"`) is silently ignored by DSM — the `additional` field will be absent from
+> every file entry.
+>
+> **`SYNO.FileStation.Stat`:** Not available on all NAS firmware versions. Do not use for
+> `get_info`; fall back to `SYNO.FileStation.List::list` with `additional=["size","time","perm","owner"]`.
+
 ---
 
 #### `get_info`