feat(selection): add selection_get — read live viewport pickfirst selection (#1)

New COM-only Entity Query tool that reads AutoCAD's implied ("pickfirst")
selection set and returns the selected entity handles + EntityInfo, so the AI
can scope work (e.g. dimension_auto) to exactly what the user picked in the
viewport instead of the whole drawing. Resolves #1 — the layer-juggling
workaround is no longer needed.

- backends/base.py: abstract selection_get() returning a status dict
- backends/com_backend.py: reads ActiveDocument.PickfirstSelectionSet, surfaces
  the PICKFIRST sysvar, handles the empty-selection case
- backends/ezdxf_backend.py: headless stub (ok=False, empty handles, same shape)
- server.py: @mcp.tool registration (readOnlyHint), SECTION 5 3 -> 4 tools
- README + CHANGELOG + pyproject: 110 -> 111 tools, v1.1.0 -> v1.1.1

Verified live on AutoCAD 2027 (COM): selection_get correctly returned the
user's grip selection across two distinct picks; 318 tests pass, ruff-clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backends/base.py

@@ -487,6 +487,26 @@ class AutoCADBackend(ABC):
     @abstractmethod
     async def analysis_select_by_type(self, entity_type: str) -> list[EntityInfo]: ...
 
+    @abstractmethod
+    async def selection_get(self) -> dict:
+        """Read the live viewport's implied ("pickfirst") selection set.
+
+        COM-only / meaningful only with live AutoCAD. Returns a dict::
+
+            {
+                "ok": bool,                # True on COM even for an empty pick
+                "count": int,
+                "handles": list[str],      # entity handles to act on
+                "entities": list[EntityInfo],  # _dc()-converted by the server
+                "pickfirst": bool | None,  # state of the PICKFIRST sysvar
+                "message" / "error": str,  # optional guidance / failure reason
+            }
+
+        The ezdxf headless backend has no viewport, so it returns ``ok=False``
+        with an empty ``handles`` list (same shape, never raises).
+        """
+        ...
+
     # ── view / screenshot ────────────────────────────────────────────────────
     @abstractmethod
     async def view_zoom_extents(self) -> dict: ...