fix(task-14): refuse CANNOSCALE on an R12 file headlessly instead of losing it at save

Measured: an R12 (AC1009) document has no OBJECTS section, so ezdxf exports
neither AcDbVariableDictionary/DICTIONARYVAR nor the seeded SCALE objects, and
drawing_save_as keeps AC1009. `_set_annotation_scale` reported `ok: True,
changed 1:1 -> 1:50` and the file reloaded as 1:1 -- the header-only write
that vanishes on reload, the class this task exists to remove, and a breach
of the backend's own R12 honesty convention (viewport_set_scale / lock).

The write is now refused with a ValueError naming R12 and "save as R2000 or
newer", after parse_scale (a malformed value is still named as such) and
before either dictionary is touched, so a refused call leaves the root
dictionary as it found it and drawing_settings still applies the sibling keys
(limits / grid / ortho survive R12). The refusal is listed with the others in
the contract and server docstrings. Pinned by two tests that save, reopen and
read the R12 file back.

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

backends/ezdxf_backend.py

@@ -6045,6 +6045,18 @@ class EzdxfBackend(AutoCADBackend):
 
     def _set_annotation_scale(self, doc, value: Any) -> str:
         name, paper, drawing = parse_scale(value)  # refused before any write
+        if self._is_r12():
+            # Measured: an R12 (AC1009) file has no OBJECTS section, so ezdxf
+            # exports neither AcDbVariableDictionary/DICTIONARYVAR nor the
+            # SCALE objects. The in-memory write "succeeded" and reloaded as
+            # 1:1 -- the header-only-write-that-vanishes class again. Refused
+            # here, before either dictionary is created, so a refused call
+            # leaves the root dictionary as it found it.
+            raise ValueError(
+                "CANNOSCALE: R12 has no OBJECTS section, so the annotation scale "
+                "(AcDbVariableDictionary and ACAD_SCALELIST) would be lost on save. "
+                "Save as R2000 or newer first."
+            )
         self._ensure_scale_entry(doc, name, paper, drawing)
         vardict = self._variable_dictionary(doc, create=True)
         entry = vardict.get("CANNOSCALE")