fix(task-17): a rotated sheet's margins turn with it, and a paper-units move is named in changed and warnings

Finding 1: page_setup_list swapped size_mm for plot_rotation 1/3 but
reported the margins un-turned, so a 297 x 210 sheet carried its portrait
margins ([20, 20, 7.5, 7.5], printable 282 x 170) while ezdxf's Page and
_paper_frame plotted [7.5, 7.5, 20, 20] (257 x 195). The mapping now lives
once in papers.turned_sheet (cross-checked against Page.from_dxf_layout
for all four rotations) and is used by the ezdxf row, _paper_frame and the
COM row. page_setup_apply resets plot_rotation to 0; headlessly it now
re-writes the turned margins un-turned so the sheet keeps the margins it
had (nothing moves, changed says nothing), and on COM, where margins come
from the .pc3, the ones that moved show up in changed.

Finding 2: both engines forced plot paper units to millimetres with no
units field in the row, so an inch-authored ANSI sheet was rescaled 25.4x
with ok: True and changed naming only the media. Rows now carry
paper_units (mm | inches | pixels, DXF group 72 / AcPlotPaperUnits);
resolve_page_setup takes paper_units="mm" (default) | "inches" | None
(keep the layout's units, as AutoCAD's own Page Setup does); a move is
reported in changed and, with its 25.4x consequence and the way back, in
the new warnings list (papers.page_setup_warnings, identical on both
engines).

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

backends/com_backend.py

@@ -1977,6 +1977,14 @@ class ComBackend(AutoCADBackend):
     # * GetPaperSize / GetPaperMargins answer in millimetres whatever
     #   PaperUnits says (ISO_A3 under PaperUnits=0 -> (420.0, 297.0); ANSI_B
     #   under 0 and 1 -> (431.8, 279.4) both times). They are never scaled.
+    #   PaperUnits itself still governs what one paper-space unit means at
+    #   the plot scale, so the row reports it (``paper_units``) and an apply
+    #   that moves it says so in ``changed`` and ``warnings``.
+    # * Both answer in the *media's* frame (the PLOTSETTINGS fields the DXF
+    #   row reads); under PlotRotation 1/3 the row turns size and margins
+    #   together through ``papers.turned_sheet``, the one mapping the headless
+    #   row and renderer use. The rotated read-back is not yet measured live
+    #   (scripts/smoke_settings_com.py, Task 24, is where it gets measured).
     # * Writing ConfigName replaces CanonicalMediaName with the new device's
     #   default when the current media does not exist on it, and writing the
     #   old device back does *not* bring the old media with it (DWG To PDF /
@@ -1989,7 +1997,6 @@ class ComBackend(AutoCADBackend):
     # rule and replays both measurements); executed live by
     # scripts/smoke_settings_com.py (Task 24).
 
-    _AC_MILLIMETERS = 1
     _AC_NO_ROTATION = 0
     #: Layout properties a ConfigName write can move as a side effect (measured
     #: live for CanonicalMediaName; units / rotation are re-applied on the same
@@ -2004,7 +2011,9 @@ class ComBackend(AutoCADBackend):
             activex_scale_label,
             center_applies,
             paper_from_size,
+            paper_units_name,
             scale_label,
+            turned_sheet,
         )
 
         def _get(attr):
@@ -2029,7 +2038,11 @@ class ComBackend(AutoCADBackend):
         except Exception as exc:
             log.debug("Layout.GetPaperMargins failed: %s", exc)
         rotation = _get("PlotRotation") or 0
-        if size and int(rotation) in (1, 3):
+        if size and margins:
+            size, margins = turned_sheet(size[0], size[1], margins, rotation)
+            size = [round(v, 2) for v in size]
+            margins = [round(v, 3) for v in margins]
+        elif size and int(rotation) in (1, 3):
             size = [size[1], size[0]]
         if _get("UseStandardScale"):
             scale = activex_scale_label(_get("StandardScale") or 0)
@@ -2055,6 +2068,7 @@ class ComBackend(AutoCADBackend):
             "plot_area": PLOT_TYPE_NAMES.get(plot_type, plot_type),
             "device": _get("ConfigName"),
             "margins_mm": margins,
+            "paper_units": paper_units_name(_get("PaperUnits")),
             "center": bool(_get("CenterPlot")) if center_applies(plot_type) else None,
         }
 
@@ -2088,7 +2102,11 @@ class ComBackend(AutoCADBackend):
         return await self._run(_sync)
 
     async def page_setup_apply(self, layout: str, setup: dict) -> dict:
-        from engineering.standards.papers import require_page_setup
+        from engineering.standards.papers import (
+            PAPER_UNITS,
+            page_setup_warnings,
+            require_page_setup,
+        )
 
         resolved = require_page_setup(setup)
         if resolved["margins_mm"] is not None:
@@ -2149,8 +2167,11 @@ class ComBackend(AutoCADBackend):
                     )
                 step = "CanonicalMediaName"
                 _set("CanonicalMediaName", media)
-                step = "PaperUnits"
-                _set("PaperUnits", self._AC_MILLIMETERS)
+                if resolved["paper_units"] is not None:
+                    # None keeps the layout's units — what AutoCAD's own Page
+                    # Setup does when a media is chosen.
+                    step = "PaperUnits"
+                    _set("PaperUnits", PAPER_UNITS[resolved["paper_units"]])
                 step = "PlotRotation"
                 _set("PlotRotation", self._AC_NO_ROTATION)
                 step = "StyleSheet"
@@ -2214,6 +2235,7 @@ class ComBackend(AutoCADBackend):
                 "layout": target.Name,
                 "applied": dict(resolved),
                 "changed": changed,
+                "warnings": page_setup_warnings(changed),
                 "plot_style_known": bool(resolved["plot_style_known"]),
                 "viewports_kept": True,
                 "backend": "com",