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/contracts/settings.py

@@ -42,7 +42,9 @@ class SettingsContract:
 
         Refusals (per key, in ``errors``; the other keys still apply): an
         unknown key; a value outside ``_SETTING_RANGES``; a malformed
-        ``limits`` / ``annotation_scale``; ``polar`` / ``polar_angle`` on the
+        ``limits`` / ``annotation_scale``; ``annotation_scale`` on an R12 file
+        headlessly (no OBJECTS section, so the value would vanish at save —
+        save as R2000 or newer first); ``polar`` / ``polar_angle`` on the
         headless engine (registry-saved — ``capability: registry_sysvar``);
         ``dimstyle`` / ``textstyle`` when the backend has no styles contract.