fix(task-25): page_setup_truth and settings_pdf_mediabox_a3 move the sheet to ANSI B before proving A3

A fresh Layout1 is already ISO A3 landscape (ezdxf R2018 default: 420 x 297,
plot_rotation 0), so an A3-only /MediaBox read-back passed with
page_setup_apply stubbed to a no-op: the A/B gate row and the v4 matrix task
could not fail on the setter. Both now apply ANSI B (432 x 279, a size no
fresh document has; spec section 9 names it) and read that PDF first, then
apply ISO A3, read its PDF, and require the setter's changed.size_mm to have
moved from B to A3. The stub fails at the first PDF.

The moved-sheet comparison is +/-0.5 mm, the spec's tolerance, not exact:
measured live on AutoCAD 2026 (scratch Documents.Add(), closed unsaved), the
COM engine reports ANSI B as 431.8 x 279.4 and the DWG To PDF.pc3 file's
MediaBox as 431.8 x 279.4 then 420.158 x 297.039 for A3; the task passes
there with changed.size_mm [[431.8, 279.4], [420.0, 297.0]]. Headless it is
[[432.0, 279.0], [420.0, 297.0]].

tests/test_benchmark_v4.py gains the mutation test the finding describes
(setter stubbed -> both rows must fail; both passed on the old code) and a
run of the gate row itself; the headless task test asserts the B stage.
README rows and the tasks_v4 docstring say what the task now does; the README
collected-tests figure follows the two new tests (3282 -> 3284). The
regenerated A/B report against v1.5.1 is byte-identical (26 -> 32/32).

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

benchmarks/correctness_suite.py

@@ -376,17 +376,45 @@ async def settings_layer_state_roundtrip():
     )
 
 
+def _sheet_near(size_mm, width_mm: float, height_mm: float) -> bool:
+    """Within the spec's ±0.5 mm: COM measures ANSI B as 431.8 × 279.4."""
+    try:
+        w, h = (float(v) for v in size_mm)
+    except (TypeError, ValueError):
+        return False
+    return abs(w - width_mm) < 0.5 and abs(h - height_mm) < 0.5
+
+
 async def settings_pdf_mediabox_a3():
-    """Page setup evidence is the PDF's own /MediaBox (spec §8.3)."""
+    """Page setup evidence is the PDF's own /MediaBox (spec §8.3).
+
+    A fresh document's Layout1 is already ISO A3 landscape, so an A3-only
+    read-back would pass with the setter stubbed out. The sheet is therefore
+    moved to ANSI B first — 432 × 279, a size no fresh document has (spec §9
+    names it) — and that PDF is read; then ISO A3 is applied and read again.
+    The A3 apply must also report the layout moved from B to A3, so the PDF
+    and the setter's own diff have to agree on where the sheet was.
+    """
     from engineering.standards.papers import resolve_page_setup
     from engineering.standards.plot import batch_plot
 
     b = await _b()
-    await b.page_setup_apply("Layout1", resolve_page_setup("ISO_A3"))
+    await b.page_setup_apply("Layout1", resolve_page_setup("ANSI_B"))
     out_dir = tempfile.mkdtemp()
-    report = await batch_plot(b, ["Layout1"], out_dir, "{drawing}-{layout}.pdf")
-    width_mm, height_mm = report["sheets"][0]["mediabox_mm"]
-    return abs(width_mm - 420.0) < 0.5 and abs(height_mm - 297.0) < 0.5
+    report_b = await batch_plot(b, ["Layout1"], out_dir, "{drawing}-{layout}-ansi_b.pdf")
+    width_b, height_b = report_b["sheets"][0]["mediabox_mm"]
+    applied = await b.page_setup_apply("Layout1", resolve_page_setup("ISO_A3"))
+    report_a3 = await batch_plot(b, ["Layout1"], out_dir, "{drawing}-{layout}-iso_a3.pdf")
+    width_a3, height_a3 = report_a3["sheets"][0]["mediabox_mm"]
+    moved = applied["changed"].get("size_mm") or [[0.0, 0.0], [0.0, 0.0]]
+    return (
+        abs(width_b - 432.0) < 0.5
+        and abs(height_b - 279.0) < 0.5
+        and _sheet_near(moved[0], 432.0, 279.0)
+        and _sheet_near(moved[1], 420.0, 297.0)
+        and abs(width_a3 - 420.0) < 0.5
+        and abs(height_a3 - 297.0) < 0.5
+    )
 
 
 CHECKS = {