fix(task-17): a layout plot never writes CenterPlot, and a refused COM write unwinds every landed one

The ActiveX Reference (CenterPlot Property, acadauto.chm, AutoCAD 2026):
"This property cannot be set to True on a layout object whose PlotType
property is set to acLayout." The shipped sequence wrote PlotType=5 and
then CenterPlot=True for the *default* call, so page_setup_apply(layout,
resolve_page_setup("ISO_A3")) could not succeed live — either COM raised
with nine writes landed and nothing put back, or the result claimed
applied.center=True over a read-back of False.

- resolve_page_setup resolves `center` to None for plot_area="layout"
  (validated as a bool first; kept as the bool for "extents"), and
  `center_applies(plot_type)` carries the rule for both read-backs.
- Both engines: page_setup_list reports `center: None` under acLayout
  instead of a stored flag nothing honours; apply never sets the centre
  bit / CenterPlot for a layout plot (ezdxf clears bit 4, the state
  AutoCAD stores for acLayout; COM clears a stale CenterPlot=True *before*
  PlotType moves to acLayout, and writes CenterPlot for an extents plot
  only *after* PlotType=acExtents).
- COM: every write is journaled and unwound in reverse when AutoCAD
  refuses one; the ValueError names the refused property and says the
  layout was restored (or what could not be). The old device-only
  rollback is the same mechanism now.
- tests: the fake enforces the documented CenterPlot rule (the shipped
  sequence fails under it, which is the reviewer's repro), and can refuse
  a named property; new cases pin the no-write, the ordering both ways,
  the reverse unwind including SetCustomScale, and the None read-backs.

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

backends/contracts/pagesetup.py

@@ -18,7 +18,10 @@ class PageSetupContract(ABC):
         orientation, plot_style, scale, plot_area, device,
         margins_mm: [top, bottom, left, right], center}``. ``paper`` is the
         catalogue name when the size matches one within 0.5 mm, otherwise the
-        media name as stored. Raises ``ValueError`` for an unknown layout.
+        media name as stored. ``center`` is a bool for an extents plot and
+        ``None`` for a layout plot, where centring is not applicable (AutoCAD
+        greys it out; ActiveX refuses ``CenterPlot = True`` under
+        ``acLayout``). Raises ``ValueError`` for an unknown layout.
         ``Model`` is never listed: model space is plotted through
         ``drawing_export_pdf(layout=None)`` and carries no sheet.
         """
@@ -33,7 +36,9 @@ class PageSetupContract(ABC):
         plot_style_known, viewports_kept}``. ``changed`` is computed from the
         read-back before and after, so re-applying the same setup reports
         ``{}``. Refuses ``Model``, an unknown layout, and a ``setup`` missing a
-        resolved key, before anything is written. Never deletes viewports.
+        resolved key, before anything is written. A write the engine refuses
+        part-way through is unwound (every landed value put back) before the
+        ``ValueError`` names the refused property. Never deletes viewports.
         """
         ...