fix(task-18): the headless Model plot is doc.modelspace(), not the current tab

EzdxfBackend.drawing_export_pdf routed layout=None / layout="Model" through
_msp(), which follows the current tab, so with a sheet current the "Model"
PDF was that sheet's paper-space content with no refusal — the same defect
finding 3 named on COM, still live on the headless engine. Measured: circle
in model space, 390x270 rectangle on an A3 sheet, sheet current — the Model
row's /MediaBox was 176.1x121.9 mm (the rectangle) instead of 121.9x121.9
(the circle). The model target is now doc.modelspace() whichever tab is
current, on both entry points.

The test that vouched for this only asserted the row's shape (paper is None,
true by construction). It is replaced by two tests that prove content: the
Model row's mediabox with a sheet current equals the one with Model current
and carries the circle's square aspect, and drawing_export_pdf(path) with no
layout does the same. Both fail without the backend change. plot.py's
docstring now says which engine does what.

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

engineering/standards/plot.py

@@ -37,7 +37,10 @@ async def batch_plot(
 
     ``layouts=None`` plots every paper-space layout; ``Model`` is plotted only
     when named, and is passed to the engine by name so the model row is model
-    space whichever tab is current. A sheet the engine cannot plot, or reports
+    space whichever tab is current — both engines honour the name (COM plots
+    the Model tab; ezdxf renders ``doc.modelspace()``, not the current tab;
+    ``tests/test_batch_plot.py`` proves it by content on the headless engine).
+    A sheet the engine cannot plot, or reports
     plotted without a file, is a per-row ``ok: False`` with its ``error``; the
     rows before it stay. Refuses (``ValueError``, before any PDF is written): an empty
     list, a layout that does not exist, a pattern with a path separator, and