docs: add shape library design spec

Defines a parametric shape library with abstract ShapeDefinition base
class and concrete subclasses for common CNC shapes (Tier 1+2).

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

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

@@ -0,0 +1,93 @@
+# Shape Library Design Spec
+
+## Overview
+
+A parametric shape library for OpenNest that provides reusable, self-describing shape classes for generating `Drawing` objects. Each shape is its own class with typed parameters, inheriting from an abstract `ShapeDefinition` base class. Inspired by PEP's WINSHAPE library.
+
+## Location
+
+- Project: `OpenNest.Core`
+- Folder: `Shapes/`
+- Namespace: `OpenNest.Shapes`
+
+## Architecture
+
+### Base Class — `ShapeDefinition`
+
+Abstract base class that all shapes inherit from.
+
+```csharp
+public abstract class ShapeDefinition
+{
+    public string Name { get; set; }
+    public abstract Drawing GetDrawing();
+
+    protected Drawing CreateDrawing(List<Entity> entities)
+    {
+        var pgm = ConvertGeometry.ToProgram(entities);
+        return new Drawing(Name, pgm);
+    }
+}
+```
+
+- `Name`: The name assigned to the resulting `Drawing`.
+- `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()`.
+
+### Shape Classes
+
+#### Tier 1 — Basics (extracted from MCP InputTools)
+
+| 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 |
+
+#### Tier 2 — Common CNC shapes
+
+| Class | Parameters | Description |
+|-------|-----------|-------------|
+| `RingShape` | `OuterDiameter`, `InnerDiameter` | Annular ring (two concentric circles) |
+| `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 |
+
+### File Structure
+
+```
+OpenNest.Core/
+  Shapes/
+    ShapeDefinition.cs
+    CircleShape.cs
+    RectangleShape.cs
+    RingShape.cs
+    RightTriangleShape.cs
+    IsoscelesTriangleShape.cs
+    TrapezoidShape.cs
+    OctagonShape.cs
+    RoundedRectangleShape.cs
+    LShape.cs
+    TShape.cs
+```
+
+### Geometry Construction
+
+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
+
+### 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.
+
+## 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