feat: add Shape Library UI with configurable shapes and flange presets

Add a Shape Library dialog (Nest > Shape Library) for creating drawings
from built-in parametric shapes. Supports configuration presets loaded
from JSON files — ships with 136 standard pipe flanges. Parameters use
TextBox inputs with architectural unit parsing (feet/inches, fractions).

- ShapeLibraryForm with split layout: shape list, preview, parameters
- ShapePreviewControl for auto-zoom rendering with info overlay
- ArchUnits utility for parsing architectural measurements
- SetPreviewDefaults() on all ShapeDefinition subclasses
- Convention-based config discovery (Configurations/{ShapeName}.json)

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

OpenNest.Core/Shapes/CircleShape.cs

@@ -7,6 +7,11 @@ namespace OpenNest.Shapes
     {
         public double Diameter { get; set; }
 
+        public override void SetPreviewDefaults()
+        {
+            Diameter = 8;
+        }
+
         public override Drawing GetDrawing()
         {
             var entities = new List<Entity>

OpenNest.Core/Shapes/FlangeShape.cs

@@ -11,6 +11,15 @@ namespace OpenNest.Shapes
         public double HolePatternDiameter { get; set; }
         public int HoleCount { get; set; }
 
+        public override void SetPreviewDefaults()
+        {
+            NominalPipeSize = 2;
+            OD = 7.5;
+            HoleDiameter = 0.875;
+            HolePatternDiameter = 5.5;
+            HoleCount = 8;
+        }
+
         public override Drawing GetDrawing()
         {
             var entities = new List<Entity>();

OpenNest.Core/Shapes/IsoscelesTriangleShape.cs

@@ -8,6 +8,12 @@ namespace OpenNest.Shapes
         public double Base { get; set; }
         public double Height { get; set; }
 
+        public override void SetPreviewDefaults()
+        {
+            Base = 8;
+            Height = 10;
+        }
+
         public override Drawing GetDrawing()
         {
             var midX = Base / 2.0;

OpenNest.Core/Shapes/LShape.cs

@@ -10,6 +10,14 @@ namespace OpenNest.Shapes
         public double LegWidth { get; set; }
         public double LegHeight { get; set; }
 
+        public override void SetPreviewDefaults()
+        {
+            Width = 8;
+            Height = 10;
+            LegWidth = 3;
+            LegHeight = 3;
+        }
+
         public override Drawing GetDrawing()
         {
             var lw = LegWidth > 0 ? LegWidth : Width / 2.0;

OpenNest.Core/Shapes/OctagonShape.cs

@@ -7,6 +7,11 @@ namespace OpenNest.Shapes
     {
         public double Width { get; set; }
 
+        public override void SetPreviewDefaults()
+        {
+            Width = 8;
+        }
+
         public override Drawing GetDrawing()
         {
             var center = Width / 2.0;

OpenNest.Core/Shapes/RectangleShape.cs

@@ -8,6 +8,12 @@ namespace OpenNest.Shapes
         public double Length { get; set; }
         public double Width { get; set; }
 
+        public override void SetPreviewDefaults()
+        {
+            Length = 12;
+            Width = 6;
+        }
+
         public override Drawing GetDrawing()
         {
             var entities = new List<Entity>

OpenNest.Core/Shapes/RightTriangleShape.cs

@@ -8,6 +8,12 @@ namespace OpenNest.Shapes
         public double Width { get; set; }
         public double Height { get; set; }
 
+        public override void SetPreviewDefaults()
+        {
+            Width = 8;
+            Height = 6;
+        }
+
         public override Drawing GetDrawing()
         {
             var entities = new List<Entity>

OpenNest.Core/Shapes/RingShape.cs

@@ -8,6 +8,12 @@ namespace OpenNest.Shapes
         public double OuterDiameter { get; set; }
         public double InnerDiameter { get; set; }
 
+        public override void SetPreviewDefaults()
+        {
+            OuterDiameter = 10;
+            InnerDiameter = 6;
+        }
+
         public override Drawing GetDrawing()
         {
             var entities = new List<Entity>

OpenNest.Core/Shapes/RoundedRectangleShape.cs

@@ -10,6 +10,13 @@ namespace OpenNest.Shapes
         public double Width { get; set; }
         public double Radius { get; set; }
 
+        public override void SetPreviewDefaults()
+        {
+            Length = 12;
+            Width = 6;
+            Radius = 1;
+        }
+
         public override Drawing GetDrawing()
         {
             var r = Radius;

OpenNest.Core/Shapes/ShapeDefinition.cs

@@ -26,6 +26,8 @@ namespace OpenNest.Shapes
 
         public abstract Drawing GetDrawing();
 
+        public virtual void SetPreviewDefaults() { }
+
         public static List<T> LoadFromJson<T>(string path) where T : ShapeDefinition
         {
             var json = File.ReadAllText(path);

OpenNest.Core/Shapes/TShape.cs

@@ -10,6 +10,14 @@ namespace OpenNest.Shapes
         public double StemWidth { get; set; }
         public double BarHeight { get; set; }
 
+        public override void SetPreviewDefaults()
+        {
+            Width = 10;
+            Height = 8;
+            StemWidth = 3;
+            BarHeight = 3;
+        }
+
         public override Drawing GetDrawing()
         {
             var sw = StemWidth > 0 ? StemWidth : Width / 3.0;

OpenNest.Core/Shapes/TrapezoidShape.cs

@@ -9,6 +9,13 @@ namespace OpenNest.Shapes
         public double BottomWidth { get; set; }
         public double Height { get; set; }
 
+        public override void SetPreviewDefaults()
+        {
+            TopWidth = 6;
+            BottomWidth = 10;
+            Height = 6;
+        }
+
         public override Drawing GetDrawing()
         {
             var offset = (BottomWidth - TopWidth) / 2.0;