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

docs/superpowers/specs/2026-09-16-v1.6-settings-design.md

@@ -118,7 +118,7 @@ name.
 - `dimstyle_modify(name, overrides)` — `changed` reports only variables that actually moved (the `hatch_edit` rule). Dimensions already using the style are listed in `dimensions_using_style` with `rerender_required: true` on the headless engine: AutoCAD re-renders them on the next regen, but a headless DIMENSION carries its rendered block, so the drawing shows the old style until the dimension is redrawn — stated rather than silent.
 - `dimstyle_set_current(name)` — `$DIMSTYLE`; the dimension tools then use it (the existing per-dimension override path stays as is).
 - `textstyle_list()`, `textstyle_create(name, font: "isocp.shx"|"isocpeur.ttf"|"arial.ttf"|"romans.shx"|<file>, height=0.0, width_factor=1.0, oblique_deg=0.0, set_current=False)`, `textstyle_set_current(name)` — `$TEXTSTYLE`; a font file that is neither a bundled preset nor present in the engine's font path is written but reported `font_resolved: false` (AutoCAD substitutes at open; the headless renderer falls back), never refused, because DXF only stores the name.
-- `mleaderstyle_list()`, `mleaderstyle_create(name, preset: "iso"|"ansi", overrides)` — arrow size, landing gap, text style, text height on ezdxf's `doc.mleader_styles`. ActiveX exposes no MLeaderStyle collection, so the COM engine refuses both with `capability: "mleaderstyle"` (the reason names `leader_create_mleader`'s existing per-leader parameters as the route); `mleaderstyle_list` on COM still reads the names from the `ACAD_MLEADERSTYLE` dictionary.
+- `mleaderstyle_list()`, `mleaderstyle_create(name, preset: "iso"|"ansi", overrides)` — arrow size, landing gap, text style, text height, both engines: ezdxf on `doc.mleader_styles`; COM through the `ACAD_MLEADERSTYLE` dictionary, whose items are `IAcadMLeaderStyle` objects (`ArrowSize` / `LandingGap` / `TextHeight` in drawing units, `TextStyle` a name — verified against the AutoCAD 2026 type library), created with `Dictionaries.Item("ACAD_MLEADERSTYLE").AddObject(name, "AcDbMLeaderStyle")` followed by the property writes. ActiveX has no `MLeaderStyles` *collection* property, which is not an engine boundary — no `mleaderstyle` capability key exists.
 - `drawing_apply_standard(standard: "iso"|"ansi", layers: bool = True, units: bool = True)` — one call: the dimstyle preset (created and set current), its text style, `drawing_settings` (units mm, decimal linear and angular units, LTSCALE 1, DIMSCALE 1; the decimal separator comes from the preset) and, with `layers`, the `mech` layer set for both standards (ANSI layer naming is company-specific; the set reuses the mech names with ISO 128 lineweights — only styles and the separator differ between the two). Returns, per item, whether it was created or was already present.
 
 ## 5. Settings (SECTION 20 shares these; 3 tools + facade growth)
@@ -187,9 +187,10 @@ A state is the layer table snapshot `{layer: {on, frozen, locked, color, linetyp
 
 ### 8.1 Capability keys (added to **both** maps)
 
-`mleaderstyle` (COM false), `dwgprops` (ezdxf false: summary fields), `dwt_write`
-(ezdxf false), `live_application`, `preferences`, `interactive_prompt` (ezdxf
-false). `tests/test_capability_contract.py` enforces both maps.
+`dwgprops` (ezdxf false: summary fields), `dwt_write` (ezdxf false),
+`live_application`, `preferences`, `interactive_prompt` (ezdxf false).
+`tests/test_capability_contract.py` enforces both maps. (`mleaderstyle` was
+listed here as COM false; the ActiveX route exists — §4.2 — so there is no key.)
 
 ### 8.2 Packs, profiles, discovery
 
@@ -219,7 +220,9 @@ shop-floor synonyms; the discovery golden set gains ≥ 10 queries for them
   (A0 841×1189 … A4 210×297; ANSI A 216×279 … E 864×1118), every catalog
   sysvar has type+meaning+engines.
 - Dual-engine tests for every tool (ezdxf real; COM fake-ActiveX asserting call
-  shapes: `DimStyles.Add`, `TextStyles.Add`, `Layout.ConfigName/…`,
+  shapes: `DimStyles.Add`, `TextStyles.Add`,
+  `Dictionaries.Item("ACAD_MLEADERSTYLE").AddObject(name, "AcDbMLeaderStyle")`,
+  `Layout.ConfigName/…`,
   `Documents.Item().Activate`, `Dictionaries.Add`+`AddXRecord`, `Views.Add`,
   `UserCoordinateSystems.Add`, `Preferences.OpenSave.*`, `SummaryInfo.*`,
   `Utility.GetPoint`).