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

server.py

@@ -5084,7 +5084,9 @@ async def drawing_settings(
     styles contract. On the live engine `annotation_scale` must name a scale in
     the drawing's scale list (SCALELISTEDIT) and AutoCAD refuses the write on a
     paper-space layout with no active viewport; the headless engine adds the
-    scale, seeding AutoCAD's default list first when the drawing has none.
+    scale, seeding AutoCAD's default list first when the drawing has none,
+    and refuses it on an R12 file (no OBJECTS section, so the value would
+    vanish at save — save as R2000 or newer first; limits/grid/ortho survive).
     Headlessly, grid/snap are stored on the active VPORT and the annotation
     scale in the variable dictionary — the places AutoCAD reads them from.
     """