refactor: merge duplicated geometry, settings-file and form helpers

Roslyn Bridge flagged these as copy-paste clusters; each pair now shares
one implementation so a fix lands everywhere at once:

- Shape.OffsetInward/OffsetOutward differed only in the target winding.
- BestFitFinder kept its own copy of SpatialQuery's arc-extreme points.
- SplitDrawingForm rebuilt DrawingSplitter's clip-region grid for preview.
- GeometryOptimizer's collinear/coradial finders (one misspelled
  GetCoradialArs) become one generic FindRelated.
- Five settings files repeated the write-with-retry loop, and two also
  repeated the unit parser and unreadable-file filter (SettingsFile).
- ClampsForm and SupportGridForm repeated their row/box builders.
- Vertical/HorizontalRemnantComparer share a RemnantComparer base.

Behavior is unchanged except that Engine/NestStorage settings loads now
also tolerate NotSupported/Security exceptions, matching the other
settings files' never-throw contract.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

OpenNest.Data/SettingsFile.cs

@@ -0,0 +1,81 @@
+using System.Text.Json;
+
+namespace OpenNest.Data;
+
+/// <summary>File handling shared by the JSON settings files under %APPDATA%\OpenNest.</summary>
+internal static class SettingsFile
+{
+    /// <summary>
+    /// Writes the file, creating the parent directory, and retries briefly on IO collisions.
+    /// The last failure propagates; the host handles a persistent write failure.
+    /// </summary>
+    public static void Write(string path, string json, int maxRetries = 3)
+    {
+        var directory = Path.GetDirectoryName(Path.GetFullPath(path));
+
+        for (var attempt = 0; attempt < maxRetries; attempt++)
+        {
+            try
+            {
+                if (!string.IsNullOrEmpty(directory))
+                    Directory.CreateDirectory(directory);
+                File.WriteAllText(path, json);
+                return;
+            }
+            catch (IOException) when (attempt < maxRetries - 1)
+            {
+                Thread.Sleep(100);
+            }
+        }
+    }
+
+    /// <summary>
+    /// Deserializes the file, or returns null when it is missing, unreadable or corrupt.
+    /// </summary>
+    public static T? Read<T>(string path, JsonSerializerOptions options)
+        where T : class
+    {
+        if (string.IsNullOrWhiteSpace(path) || !File.Exists(path))
+            return null;
+
+        try
+        {
+            return JsonSerializer.Deserialize<T>(File.ReadAllText(path), options);
+        }
+        catch (Exception ex) when (IsUnreadableFile(ex))
+        {
+            return null;
+        }
+    }
+
+    public static bool IsUnreadableFile(Exception ex) =>
+        ex
+            is JsonException
+                or IOException
+                or UnauthorizedAccessException
+                or NotSupportedException
+                or System.Security.SecurityException;
+
+    /// <summary>
+    /// Accepts only a defined unit name (case-insensitive). Enum.TryParse would also accept
+    /// numeric strings ("7") and comma-joined names, producing undefined or unintended values.
+    /// </summary>
+    public static bool TryParseUnits(JsonElement? element, out Units units)
+    {
+        units = default;
+        if (element is not { ValueKind: JsonValueKind.String } value)
+            return false;
+
+        var text = value.GetString();
+        foreach (var candidate in Enum.GetValues<Units>())
+        {
+            if (string.Equals(candidate.ToString(), text, StringComparison.OrdinalIgnoreCase))
+            {
+                units = candidate;
+                return true;
+            }
+        }
+
+        return false;
+    }
+}