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

backends/ezdxf_backend.py

@@ -830,8 +830,18 @@ def _sync_header_to_dimstyle(doc, style) -> None:
     `Standard` reproduces the header `drawing_new` made. `DIMTXSTY` has no
     schema default; AutoCAD reads ``Standard`` for it, as `_dimstyle_value`
     reports.
+
+    The arrowheads cross a vocabulary boundary here. The DIMSTYLE entry holds
+    ezdxf's writer spelling (``OBLIQUE``, see `_write_dimstyle_values`); the
+    ``$DIMBLK`` / ``$DIMBLK1`` / ``$DIMBLK2`` header variables are DXF-facing
+    and spell a built-in with its underscore (``_OBLIQUE``, what a live seat
+    stores and what `system_get_variable("DIMBLK")` returns on COM). Copied
+    raw, the saved file carried ``$DIMBLK "OBLIQUE"`` next to blocks named
+    ``_OBLIQUE`` — a current-override naming a block that does not exist —
+    and the two engines answered the same read two ways. `reported_arrowhead`
+    is the same read-side rule `_dimstyle_value` applies.
     """
-    from engineering.standards.dimstyles import DIM_VARIABLE_WHITELIST
+    from engineering.standards.dimstyles import DIM_VARIABLE_WHITELIST, reported_arrowhead
 
     for var in sorted(DIM_VARIABLE_WHITELIST):
         attr = var.lower()
@@ -840,6 +850,8 @@ def _sync_header_to_dimstyle(doc, style) -> None:
             value = _dimstyle_schema_default(var)
         if value is None and var == "DIMTXSTY":
             value = "Standard"
+        if var in _ARROWHEAD_VARS:
+            value = reported_arrowhead(value)
         key = f"${var}"
         if value is None:
             try:
@@ -1342,7 +1354,6 @@ class EzdxfBackend(AutoCADBackend):
                     False, reason="acis_generation_requires_live_autocad"
                 ),
                 "lisp": FeatureCapability(False, reason="live_com_only"),
-                "mleaderstyle": FeatureCapability(True, "native"),
             },
         )
 
@@ -6079,38 +6090,29 @@ class EzdxfBackend(AutoCADBackend):
         return await self._async(_sync)
 
     async def mleaderstyle_create(self, name: str, values: dict) -> dict:
-        from engineering.standards.mleaderstyles import MLEADER_KEYS
+        from engineering.standards.mleaderstyles import validate_mleaderstyle
         from security import sanitize_symbol_name
 
         clean = sanitize_symbol_name(name, kind="mleaderstyle")
-        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"
-            )
+        typed = validate_mleaderstyle(values)
 
         def _sync():
             doc = self._require_doc()
             if doc.mleader_styles.has_entry(clean):
                 raise ValueError(f"mleaderstyle_create: leader style {clean!r} already exists")
             text_style_name, textstyle_created = _ensure_textstyle(
-                doc, str(values["text_style"]), refusal_key="text_style"
+                doc, typed["text_style"], refusal_key="text_style"
             )
             style = doc.mleader_styles.new(clean)
-            style.dxf.arrow_head_size = float(values["arrow_size"])
-            style.dxf.landing_gap_size = float(values["landing_gap"])
-            style.dxf.char_height = float(values["text_height"])
+            style.dxf.arrow_head_size = typed["arrow_size"]
+            style.dxf.landing_gap_size = typed["landing_gap"]
+            style.dxf.char_height = typed["text_height"]
             style.dxf.text_style_handle = doc.styles.get(text_style_name).dxf.handle
             self._mark_dirty()
             return {
                 "ok": True,
                 "name": clean,
-                "values": {
-                    "arrow_size": float(values["arrow_size"]),
-                    "landing_gap": float(values["landing_gap"]),
-                    "text_style": text_style_name,
-                    "text_height": float(values["text_height"]),
-                },
+                "values": {**typed, "text_style": text_style_name},
                 "textstyle_created": textstyle_created,
             }