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

engineering/standards/papers.py

@@ -13,6 +13,16 @@ tables of two enums that do not agree with each other:
 
 A scale without a code on an engine goes through the custom numerator /
 denominator on that engine, and the read-back reports the same label either way.
+
+Centring belongs to an *extents* plot only. 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" — a layout plot is
+the whole sheet from its own origin, and the Page Setup dialog greys the box
+out. ``resolve_page_setup`` therefore resolves ``center`` to ``None`` (not
+applicable, never written, read back as ``None``) when ``plot_area`` is
+``layout``; the bool is kept only for ``extents``. Both engines follow the
+same resolved value, so the DXF never carries a centre bit AutoCAD would
+ignore and the live engine never issues the write AutoCAD refuses.
 """
 
 from __future__ import annotations
@@ -28,11 +38,13 @@ __all__ = [
     "PAPER_INCHES",
     "PAPER_SIZES",
     "PLOT_AREAS",
+    "PLOT_TYPE_LAYOUT",
     "PLOT_TYPE_NAMES",
     "REQUIRED_SETUP_KEYS",
     "SCALES",
     "activex_scale_label",
     "canonical_media_name",
+    "center_applies",
     "dxf_scale_label",
     "paper_from_size",
     "paper_size_mm",
@@ -135,6 +147,8 @@ _ACTIVEX_SCALE_LABEL = {code: label for label, code in ACTIVEX_PLOT_SCALE.items(
 
 #: Plot area → DXF/ActiveX plot type (``AcPlotType``: acExtents=1, acLayout=5).
 PLOT_AREAS: dict[str, int] = {"layout": 5, "extents": 1}
+#: ``AcPlotType.acLayout`` — the plot type under which centring does not apply.
+PLOT_TYPE_LAYOUT = 5
 PLOT_TYPE_NAMES: dict[int, str] = {
     0: "display",
     1: "extents",
@@ -243,6 +257,19 @@ def plot_style_known(name) -> bool:
     return str(name or "").strip().lower() in {entry.lower() for entry in CTB_CATALOG}
 
 
+def center_applies(plot_type) -> bool:
+    """Whether "centre the plot" means anything under this ``AcPlotType``.
+
+    False for ``acLayout`` (5): the sheet is plotted from its origin, AutoCAD
+    greys the option out and ActiveX refuses ``CenterPlot = True``. Read-backs
+    report ``center: None`` there instead of a stored flag nothing honours.
+    """
+    try:
+        return int(plot_type) != PLOT_TYPE_LAYOUT
+    except (TypeError, ValueError):
+        return False
+
+
 def resolve_page_setup(
     paper: str,
     orientation: str = "landscape",
@@ -256,7 +283,10 @@ def resolve_page_setup(
     """Validate a page setup request and resolve it to what both engines write.
 
     Raises ``ValueError`` naming the offending field; nothing here touches a
-    drawing. ``margins_mm`` is ``[top, bottom, left, right]``.
+    drawing. ``margins_mm`` is ``[top, bottom, left, right]``. ``center`` is
+    validated as a bool and resolved to ``None`` for ``plot_area="layout"``,
+    where centring is not applicable (see the module docstring) — the default
+    call (layout, centre) is therefore a layout plot with no centring write.
     """
     name = _paper_key(paper)
     width, height = paper_size_mm(name, _orientation(orientation))
@@ -300,6 +330,7 @@ def resolve_page_setup(
 
     if not isinstance(center, bool):
         raise ValueError(f"center: must be true or false, got {center!r}")
+    plot_type = PLOT_AREAS[plot_area]
 
     return {
         "paper": name,
@@ -314,10 +345,10 @@ def resolve_page_setup(
         "dxf_standard_scale_type": DXF_STANDARD_SCALE_TYPE.get(label),
         "activex_standard_scale": ACTIVEX_PLOT_SCALE.get(label),
         "plot_area": plot_area,
-        "plot_type": PLOT_AREAS[plot_area],
+        "plot_type": plot_type,
         "device": device_name,
         "margins_mm": margins,
-        "center": center,
+        "center": center if center_applies(plot_type) else None,
     }