fix(task-23): user_select reports the picked point in WCS — GetEntity answers in the current UCS, so translate acUCS→acWorld before returning

Measured on AutoCAD 2026: under a UCS at (100,50) rotated 90 deg, a pick on a
circle at WCS (105,70) came back from IAcadUtility.GetEntity as (20,-5,0),
while Utility.GetPoint already answered in WCS. A model doing ucs_set ->
user_select -> entity_create_* at `picked` would place geometry at a silently
wrong position. The point goes through Utility.TranslateCoordinates(acUCS,
acWorld) packed as a VT_ARRAY|VT_R8 VARIANT (the plain tuple GetEntity hands
back is refused as "Invalid argument Point"). The fake Utility now models the
UCS frame and the VARIANT requirement; a new test pins the measured figures
and checks GetPoint stays untranslated. Docstrings state WCS.

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

backends/contracts/environment.py

@@ -148,7 +148,7 @@ class EnvironmentContract(ABC):
         reason="prompting the operator needs a live AutoCAD session",
     )
     async def user_select(self, prompt: str, mode: str = "single") -> dict:
-        """→ ``{"cancelled", "handles": [...]}``."""
+        """→ ``{"cancelled", "handles": [...]}``; ``mode="single"`` adds ``"picked": [x, y]`` in WCS."""
         ...
 
     @capability(  # noqa: B027 — @capability supplies the body