fix(task-10): $DIMBLK header carries the canonical arrowhead name; MLeaderStyle is authored on COM through the ACAD_MLEADERSTYLE dictionary

Two review findings, both root causes.

1. `_sync_header_to_dimstyle` copied the DIMSTYLE entry's ezdxf-internal
   arrowhead spelling (OBLIQUE) into the DXF-facing $DIMBLK/$DIMBLK1/$DIMBLK2
   header, so the saved file named a block it did not hold (the blocks are
   _OBLIQUE / _DOT) and `system_get_variable("DIMBLK")` answered "OBLIQUE"
   headlessly against "_OBLIQUE" on COM and in the same engine's own
   `dimstyle_list` row. The header write now goes through
   `reported_arrowhead` — the same read-side rule `_dimstyle_value` applies.
   Pinned by header, `system_get_variable`, save/reopen (`ezdxf.readfile`
   header equals the block the file holds) and the Restore-to-Standard
   empty-string case in `test_builtin_arrowheads_render_save_and_report_one_spelling`.

2. The `mleaderstyle` capability refusal rested on a false engine boundary.
   acax25enu.tlb (AutoCAD 2026) exposes IAcadMLeaderStyle with Name,
   ArrowSize, LandingGap, TextHeight, TextStyle, ... — exactly what
   `Dictionaries.Item("ACAD_MLEADERSTYLE").Item(i)` returns — and
   IAcadDictionary.AddObject(keyword, "AcDbMLeaderStyle") creates one.
   COM `mleaderstyle_list` now reads the four values off those objects
   (`values_available: True`); COM `mleaderstyle_create` validates every value
   (`validate_mleaderstyle`, shared with ezdxf), checks the clash and the text
   style before AddObject, then writes the four properties. The contract
   method is `@abstractmethod` again, the key is gone from both capability
   maps, the fake ActiveX surface models the real call shape, and the spec
   (§4.2, §8.1, §9) states the ActiveX route.

Live AutoCAD was not running on this machine (GetActiveObject: operation
unavailable; no acad.exe), so the COM route is type-library-verified and
fake-tested, not executed live — scripts/smoke_settings_com.py (Task 24)
owns that.

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

engineering/standards/mleaderstyles.py

@@ -2,8 +2,10 @@
 
 Sized to match the dimension presets they sit beside (ISO-25: 2.5 mm arrows
 and lettering; ANSI: 3 mm), so a leader note and a dimension on the same
-sheet read at one size. Only ezdxf can author an MLEADERSTYLE object;
-ActiveX exposes no collection for it (see ``backends/contracts/styles.py``).
+sheet read at one size. Both engines author the style: ezdxf on
+``doc.mleader_styles``, ActiveX through the ``ACAD_MLEADERSTYLE`` dictionary's
+``AddObject(name, "AcDbMLeaderStyle")`` and the ``IAcadMLeaderStyle``
+properties (see ``backends/contracts/styles.py``).
 """
 
 from __future__ import annotations
@@ -13,7 +15,7 @@ from typing import Any
 
 from engineering.standards.names import check_name
 
-__all__ = ["MLEADER_KEYS", "MLEADER_PRESETS", "resolve_mleaderstyle"]
+__all__ = ["MLEADER_KEYS", "MLEADER_PRESETS", "resolve_mleaderstyle", "validate_mleaderstyle"]
 
 MLEADER_PRESETS: dict[str, dict[str, Any]] = {
     "iso": {"arrow_size": 2.5, "landing_gap": 1.0, "text_style": "ISOCP", "text_height": 2.5},
@@ -57,3 +59,27 @@ def resolve_mleaderstyle(preset: str, overrides: dict | None = None) -> dict[str
         else:
             values[name] = _positive(name, value)
     return values
+
+
+def validate_mleaderstyle(values: Any) -> dict[str, Any]:
+    """The four ``MLEADER_KEYS`` of ``values``, typed, or a refusal naming the key.
+
+    What both engines' ``mleaderstyle_create`` run before touching a table:
+    a missing key is ``ValueError`` (``resolve a preset first``), and each
+    value is checked exactly as :func:`resolve_mleaderstyle` checks an
+    override — so a hand-built dict is held to the preset's rules too.
+    Unknown keys are ignored: the row shape is the contract, not the input.
+    """
+    if not isinstance(values, dict):
+        raise TypeError(f"values: expected a mapping, got {type(values).__name__}")
+    missing = [key for key in MLEADER_KEYS if key not in values]
+    if missing:
+        raise ValueError(
+            f"mleaderstyle_create: values is missing {missing}; resolve a preset first"
+        )
+    return {
+        "arrow_size": _positive("arrow_size", values["arrow_size"]),
+        "landing_gap": _positive("landing_gap", values["landing_gap"], allow_zero=True),
+        "text_style": check_name("text_style", values["text_style"], what="text style name"),
+        "text_height": _positive("text_height", values["text_height"]),
+    }