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/ezdxf_backend.py

@@ -2336,6 +2336,7 @@ class EzdxfBackend(AutoCADBackend):
     def _page_setup_row(layout) -> dict:
         from engineering.standards.papers import (
             PLOT_TYPE_NAMES,
+            center_applies,
             dxf_scale_label,
             paper_from_size,
             scale_label,
@@ -2355,6 +2356,7 @@ class EzdxfBackend(AutoCADBackend):
             scale = dxf_scale_label(int(dxf.standard_scale_type), numerator, denominator)
         else:
             scale = scale_label(numerator, denominator)
+        plot_type = int(dxf.plot_type)
         return {
             "layout": layout.name,
             "paper": paper_from_size(width, height) or media,
@@ -2363,7 +2365,7 @@ class EzdxfBackend(AutoCADBackend):
             "orientation": "landscape" if width >= height else "portrait",
             "plot_style": str(dxf.current_style_sheet),
             "scale": scale,
-            "plot_area": PLOT_TYPE_NAMES.get(int(dxf.plot_type), str(dxf.plot_type)),
+            "plot_area": PLOT_TYPE_NAMES.get(plot_type, str(dxf.plot_type)),
             "device": str(dxf.plot_configuration_file),
             "margins_mm": [
                 round(float(dxf.top_margin), 3),
@@ -2371,7 +2373,9 @@ class EzdxfBackend(AutoCADBackend):
                 round(float(dxf.left_margin), 3),
                 round(float(dxf.right_margin), 3),
             ],
-            "center": bool(flags & layout.PLOT_CENTERED),
+            # A layout plot has no centring (AutoCAD greys it out; ActiveX
+            # refuses CenterPlot=True under acLayout): None, not a bit nobody honours.
+            "center": bool(flags & layout.PLOT_CENTERED) if center_applies(plot_type) else None,
         }
 
     @staticmethod
@@ -2474,6 +2478,9 @@ class EzdxfBackend(AutoCADBackend):
                 dxf.standard_scale_type = 16
                 dxf.scale_numerator, dxf.scale_denominator = numerator, denominator
                 target.use_standard_scale(False)
+            # resolve_page_setup hands over None for a layout plot: the bit is
+            # cleared, never set — the state AutoCAD itself stores for acLayout,
+            # and the mirror of the live engine never writing CenterPlot there.
             target.plot_centered(bool(resolved["center"]))
             # Limits follow the paper; viewports are deliberately left alone
             # (Paperspace.page_setup() would delete them).