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

engineering/environment/ucs.py

@@ -12,6 +12,34 @@ import math
 WORLD = "world"
 ORTHO_TOLERANCE_DEG = 1e-3
 
+#: The WCS frame — what ``$UCSORG`` / ``$UCSXDIR`` / ``$UCSYDIR`` (and the
+#: UCSORG / UCSXDIR / UCSYDIR system variables) hold while no UCS is active.
+WORLD_ORIGIN = (0.0, 0.0, 0.0)
+WORLD_X_AXIS = (1.0, 0.0, 0.0)
+WORLD_Y_AXIS = (0.0, 1.0, 0.0)
+WORLD_TOLERANCE = 1e-9
+
+
+def is_world_axes(origin, x_axis, y_axis, *, tol: float = WORLD_TOLERANCE) -> bool:
+    """True when the frame *is* the WCS — the test ``$UCSNAME == ""`` cannot make.
+
+    An unnamed UCS (``UCS Origin`` / ``UCS 3P`` without saving — the common
+    kind) also has an empty name, so "is the drawing in WCS" has to be
+    answered from the frame itself: AutoCAD's own answer is WORLDUCS, and
+    this is the same comparison for a header that has no such variable.
+    """
+    for got, want in (
+        (origin, WORLD_ORIGIN),
+        (x_axis, WORLD_X_AXIS),
+        (y_axis, WORLD_Y_AXIS),
+    ):
+        coords = tuple(float(c) for c in got)
+        if len(coords) != 3:
+            return False
+        if any(abs(a - b) > tol for a, b in zip(coords, want, strict=True)):
+            return False
+    return True
+
 
 def _vec3(value, where: str) -> tuple[float, float, float]:
     if not isinstance(value, (list, tuple)) or len(value) != 3: