docs: address spec review feedback for shape library

Add null safety in CreateDrawing helper, default Name from class name,
clarify diameter-vs-radius conventions, octagon width definition,
arc direction for rounded rectangles, and MCP parameter mapping.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/superpowers/specs/2026-03-17-shape-library-design.md

@@ -14,25 +14,40 @@ A parametric shape library for OpenNest that provides reusable, self-describing
 
 ### Base Class — `ShapeDefinition`
 
-Abstract base class that all shapes inherit from.
+Abstract base class that all shapes inherit from. `Name` defaults to the shape type name (e.g. `"Rectangle"`) but can be overridden.
 
 ```csharp
 public abstract class ShapeDefinition
 {
     public string Name { get; set; }
+
+    protected ShapeDefinition()
+    {
+        // Default name to the concrete class name, stripping "Shape" suffix
+        var typeName = GetType().Name;
+        Name = typeName.EndsWith("Shape")
+            ? typeName.Substring(0, typeName.Length - 5)
+            : typeName;
+    }
+
     public abstract Drawing GetDrawing();
 
     protected Drawing CreateDrawing(List<Entity> entities)
     {
         var pgm = ConvertGeometry.ToProgram(entities);
+
+        if (pgm == null)
+            throw new InvalidOperationException(
+                $"Failed to create program for shape '{Name}'. Check that parameters produce valid geometry.");
+
         return new Drawing(Name, pgm);
     }
 }
 ```
 
-- `Name`: The name assigned to the resulting `Drawing`.
+- `Name`: The name assigned to the resulting `Drawing`. Defaults to the shape class name without the "Shape" suffix. Never null.
 - `GetDrawing()`: Each shape implements this to build its geometry and return a `Drawing`.
-- `CreateDrawing()`: Shared helper that converts a list of geometry entities into a `Drawing` via `ConvertGeometry.ToProgram()`.
+- `CreateDrawing()`: Shared helper that converts a list of geometry entities into a `Drawing` via `ConvertGeometry.ToProgram()`. Throws `InvalidOperationException` if the geometry is degenerate (prevents null `Program` from reaching `Drawing.UpdateArea()`).
 
 ### Shape Classes
 
@@ -41,20 +56,20 @@ public abstract class ShapeDefinition
 | Class | Parameters | Description |
 |-------|-----------|-------------|
 | `RectangleShape` | `Width`, `Height` | Axis-aligned rectangle from origin |
-| `CircleShape` | `Diameter` | Circle centered at origin |
-| `LShape` | `Width`, `Height` | L-shaped profile — full width bottom, half-width top-left |
-| `TShape` | `Width`, `Height` | T-shaped profile — full-width top bar, centered stem |
+| `CircleShape` | `Diameter` | Circle centered at origin. Implementation divides by 2 for the `Circle` entity's radius parameter. |
+| `LShape` | `Width`, `Height`, `LegWidth`?, `LegHeight`? | L-shaped profile. `LegWidth` defaults to `Width/2`, `LegHeight` defaults to `Height/2`. |
+| `TShape` | `Width`, `Height`, `StemWidth`?, `BarHeight`? | T-shaped profile. `StemWidth` defaults to `Width/3`, `BarHeight` defaults to `Height/3`. |
 
 #### Tier 2 — Common CNC shapes
 
 | Class | Parameters | Description |
 |-------|-----------|-------------|
-| `RingShape` | `OuterDiameter`, `InnerDiameter` | Annular ring (two concentric circles) |
+| `RingShape` | `OuterDiameter`, `InnerDiameter` | Annular ring (two concentric circles). Both converted to radius internally. |
 | `RightTriangleShape` | `Width`, `Height` | Right triangle with the right angle at origin |
 | `IsoscelesTriangleShape` | `Base`, `Height` | Isosceles triangle centered on base |
 | `TrapezoidShape` | `TopWidth`, `BottomWidth`, `Height` | Trapezoid with bottom edge centered under top |
-| `OctagonShape` | `Width` | Regular octagon fitting within the given width |
-| `RoundedRectangleShape` | `Width`, `Height`, `Radius` | Rectangle with arc corners |
+| `OctagonShape` | `Width` | Regular octagon where `Width` is the flat-to-flat distance |
+| `RoundedRectangleShape` | `Width`, `Height`, `Radius` | Rectangle with 90-degree CCW arc corners |
 
 ### File Structure
 
@@ -78,16 +93,19 @@ OpenNest.Core/
 
 Each shape builds a `List<Entity>` (using `Line`, `Arc`, `Circle` from `OpenNest.Geometry`) and passes it to the base `CreateDrawing()` helper. Shapes are constructed at the origin (0,0) with positive X/Y extents.
 
-- **Lines** for straight edges
-- **Arcs** for rounded corners (RoundedRectangleShape) and ring segments
-- **Circles** for CircleShape and RingShape outer/inner boundaries
+- **Lines** for straight edges — endpoints must chain end-to-end for `ShapeBuilder` to detect closed shapes.
+- **Arcs** for rounded corners (`RoundedRectangleShape`). Arcs use CCW direction (not reversed) with angles in radians.
+- **Circles** for `CircleShape` and `RingShape` outer/inner boundaries.
 
 ### MCP Integration
 
-`InputTools.CreateDrawing` in `OpenNest.Mcp` will be refactored to instantiate the appropriate `ShapeDefinition` subclass and call `GetDrawing()`, replacing the existing private `CreateRectangle`, `CreateCircle`, `CreateLShape`, `CreateTShape` methods.
+`InputTools.CreateDrawing` in `OpenNest.Mcp` will be refactored to instantiate the appropriate `ShapeDefinition` subclass and call `GetDrawing()`, replacing the existing private `CreateRectangle`, `CreateCircle`, `CreateLShape`, `CreateTShape` methods. The MCP tool's existing flat parameter names (`radius`, `width`, `height`) are mapped to the shape class properties at the MCP layer. The `gcode` case remains as-is.
+
+New Tier 2 shapes can be exposed via the MCP tool by extending the `shape` parameter's accepted values and mapping to the new shape classes, with additional MCP parameters as needed.
 
 ## Future Expansion
 
 - Additional shapes (Tier 3): Single-D, Parallelogram, House, Stair, Rectangle with chamfer(s), Ring segment, Slot rectangle
 - UI shape picker with per-shape parameter editors
 - Shape discovery via reflection or static registry
+- `LShape`/`TShape` additional sub-dimension parameters for full parametric control