Add AutoCAD MCP premium skill and backend APIs

Introduce the autocad-mcp-premium skill with workflow docs, examples, and references (.claude/skills/autocad-mcp-premium/*). Add PlanSpec/critique plumbing and engineering helpers (engineering/plan_spec.py, engineering/critique.py) plus tests and a smoke script. Extend backend API (backends/base.py) with corner operations and premium meta-tools (trim/extend/fillet/chamfer, drawing_plan, drawing_critique, point_from_snap, construction_xline/clear, drawing_apply_iso_layers, dimension_auto, entity_select_smart) and implement COM backend corner ops (backends/com_backend.py) with placeholders for meta-tools. Update CLAUDE.md to document the premium drawing rules and standard workflow. Misc: add examples, references, and tests for premium features, and minor server/backend adjustments.

engineering/__init__.py

@@ -21,6 +21,17 @@ from .layers import (
     ensure_engineering_layers,
     ensure_standard_linetypes,
 )
+from .plan_spec import (
+    ALL_CRITIQUE_FOCUSES,
+    ISO_128_LINEWEIGHTS_MM,
+    CritiqueFocus,
+    DimStyle,
+    Issue,
+    LayerSetId,
+    PlanSpec,
+    SheetSize,
+    SnapType,
+)
 from .section import apply_section_hatch, cutting_plane_line
 
 # Agent B contributes titleblock + validator. Imports are guarded so this
@@ -62,4 +73,13 @@ __all__ = [
     "DrawingValidator",
     "ValidationResult",
     "ValidationFinding",
+    "PlanSpec",
+    "Issue",
+    "CritiqueFocus",
+    "SheetSize",
+    "LayerSetId",
+    "DimStyle",
+    "SnapType",
+    "ALL_CRITIQUE_FOCUSES",
+    "ISO_128_LINEWEIGHTS_MM",
 ]