fix(task-22): ucs_list decides "world" from the frame, not the empty name — an unnamed UCS is its own current row

An unnamed UCS (UCS Origin / 3P without saving, the common kind) has an
empty UCSNAME exactly like WCS does, so both engines answered
`current: world` while the drawing was not in WCS — verified live on
AutoCAD 2026 (`_.UCS _O 10,10,0` → UCSNAME="" WORLDUCS=0) and on the DXF
it writes ($UCSNAME='' $UCSORG=(0,20,0)). A silent wrong value.

- COM: a named match still wins; otherwise WORLDUCS (AutoCAD's own answer)
  decides, and WORLDUCS=0 appends a `name: None` row built from
  UCSORG/UCSXDIR/UCSYDIR.
- ezdxf: the same rule against $UCSORG/$UCSXDIR/$UCSYDIR via the new
  `engineering.environment.ucs.is_world_axes`; a stale $UCSNAME falls back
  to the frame instead of leaving no row current.
- Server `ucs_list` result: `current` is null for an unnamed UCS (never
  `world`), plus `current_unnamed: bool` so null is unambiguous.
- Tests: helper, headless (unnamed row, stale name, and the exact AutoCAD
  DXF over the wire), and COM fake reading WORLDUCS + the frame variables.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FKFStjEFzpGyLq5bMpWxTd

backends/contracts/environment.py

@@ -86,7 +86,14 @@ class EnvironmentContract(ABC):
 
     @abstractmethod
     async def ucs_list(self) -> list[dict]:
-        """→ ``[{"name", "origin", "x_axis", "y_axis", "current": bool}]`` + the implicit "world"."""
+        """→ ``[{"name", "origin", "x_axis", "y_axis", "current": bool}]`` + the implicit "world".
+
+        Exactly one row is current. An *unnamed* current UCS (``UCS Origin`` /
+        ``3P`` without saving) is a trailing row with ``name: None`` carrying
+        its frame — "world" is current only when the frame is the WCS
+        (WORLDUCS live, the ``$UCS*`` header headless), never because the
+        name is empty.
+        """
         ...
 
     @abstractmethod