fix(ui): format stock sizes invariantly and bound validation notices

Address Task 3b quality review:
- FormatSize now emits invariant decimal notation so a comma-decimal
  locale cannot produce stock text its own invariant parser rejects.
- Validation notices embed a bounded single-line preview instead of the
  full cell value.
- Documented GetPlateOptions' all-or-nothing throwing contract.
- STA test helpers run the worker as a background thread with a
  diagnostic timeout message; added a de-DE fractional stock round-trip
  regression.

OpenNest.WinForms.Tests/ArchUnitsTests.cs

@@ -55,10 +55,11 @@ public class ArchUnitsTests
         {
             try { action(); }
             catch (Exception ex) { error = ex; }
-        });
+        })
+        { IsBackground = true };
         thread.SetApartmentState(ApartmentState.STA);
         thread.Start();
-        Assert.True(thread.Join(TimeSpan.FromSeconds(15)));
+        Assert.True(thread.Join(TimeSpan.FromSeconds(15)), "The STA test did not complete.");
         if (error != null)
             ExceptionDispatchInfo.Capture(error).Throw();
     }

OpenNest.WinForms.Tests/Forms/AutoNestFormStockOptionsTests.cs

@@ -1,5 +1,6 @@
 using System.Collections;
 using System.ComponentModel;
+using System.Globalization;
 using System.Reflection;
 using System.Runtime.ExceptionServices;
 using System.Text.RegularExpressions;
@@ -160,6 +161,31 @@ public class AutoNestFormStockOptionsTests
         grid.CancelEdit();
     });
 
+    [Fact]
+    public void FractionalStockSizesRoundTripUnderCommaDecimalCulture() => RunSta(() =>
+    {
+        var original = CultureInfo.CurrentCulture;
+        try
+        {
+            CultureInfo.CurrentCulture = CultureInfo.GetCultureInfo("de-DE");
+            using var form = new StockTestForm();
+            // FormatSize must emit invariant dots or the reloaded collection would be
+            // rejected as invalid under the invariant parser.
+            form.LoadPlateOptions(new List<PlateOption>
+            {
+                new() { Width = 48.5, Length = 96.25, Cost = 25 },
+            }, 0.5);
+            Assert.True(form.TryGetPlateOptions(out var options, out var error));
+            Assert.Null(error);
+            Assert.Equal(48.5, options[0].Width);
+            Assert.Equal(96.25, options[0].Length);
+        }
+        finally
+        {
+            CultureInfo.CurrentCulture = original;
+        }
+    });
+
     private sealed class StockTestForm : AutoNestForm
     {
         public StockTestForm() : base(new Nest()) { }
@@ -212,10 +238,11 @@ public class AutoNestFormStockOptionsTests
         {
             try { action(); }
             catch (Exception ex) { error = ex; }
-        });
+        })
+        { IsBackground = true };
         thread.SetApartmentState(ApartmentState.STA);
         thread.Start();
-        Assert.True(thread.Join(TimeSpan.FromSeconds(15)));
+        Assert.True(thread.Join(TimeSpan.FromSeconds(15)), "The STA test did not complete.");
         if (error != null)
             ExceptionDispatchInfo.Capture(error).Throw();
     }

OpenNest/Forms/AutoNestForm.cs

@@ -198,6 +198,11 @@ namespace OpenNest.Forms
             return nestItems;
         }
 
+        /// <summary>
+        /// Returns the validated stock collection. Throws <see cref="FormatException"/> when a
+        /// nonblank row is invalid or timed out; all-or-nothing, never a valid prefix. Prefer
+        /// <see cref="TryGetPlateOptions"/> for callers that must report the failure.
+        /// </summary>
         public List<PlateOption> GetPlateOptions()
         {
             if (!TryGetPlateOptions(out var options, out var error))
@@ -212,7 +217,7 @@ namespace OpenNest.Forms
             if (!plateGrid.EndEdit())
             {
                 var value = plateGrid.EditingControl?.Text ?? plateGrid.CurrentCell?.Value?.ToString();
-                error = $"Invalid stock size '{value}'. Enter positive dimensions as W x L.";
+                error = $"Invalid stock size '{Preview(value)}'. Enter positive dimensions as W x L.";
                 return false;
             }
 
@@ -228,7 +233,7 @@ namespace OpenNest.Forms
                     continue;
                 if (!TryParseSize(item.Size, out var width, out var length))
                 {
-                    error = $"Invalid stock size '{item.Size}' in row {index + 1}. Enter positive dimensions as W x L.";
+                    error = $"Invalid stock size '{Preview(item.Size)}' in row {index + 1}. Enter positive dimensions as W x L.";
                     if (index < plateGrid.Rows.Count)
                         plateGrid.Rows[index].ErrorText = error;
                     return false;
@@ -261,6 +266,16 @@ namespace OpenNest.Forms
             MessageBox.Show(this, error, "Auto Nest", MessageBoxButtons.OK, MessageBoxIcon.Warning);
         }
 
+        // Bounded single-line preview so a long or multiline cell value cannot produce an
+        // unreadable message box; the full value stays in the editable cell.
+        private static string Preview(string value)
+        {
+            if (string.IsNullOrEmpty(value))
+                return string.Empty;
+            var line = value.Replace("\r", " ").Replace("\n", " ");
+            return line.Length <= 40 ? line : line[..40] + "...";
+        }
+
         public void LoadPlateOptions(List<PlateOption> options, double salvageRate)
         {
             if (options != null && options.Count > 0)
@@ -396,7 +411,9 @@ namespace OpenNest.Forms
 
         private static string FormatSize(double width, double length)
         {
-            return $"{width:G} x {length:G}";
+            // Invariant: TryParseSize requires invariant decimal notation; current-culture
+            // formatting would produce comma decimals that the parser then rejects.
+            return FormattableString.Invariant($"{width:G} x {length:G}");
         }
 
         private void PartsGrid_DataError(object sender, DataGridViewDataErrorEventArgs e)