feat(io): add Gravograph .CHR font reader with text-to-geometry

Add ChrFont, a reader for Gravograph .CHR engraving fonts, plus UI to
convert placed text into engraved geometry in the CAD converter.

The .CHR files are obfuscated with a single-byte XOR. Different
GravoStyle releases use different keys (0x2F in older versions, 0xCF in
the 7000 series, and others across the font library), so the key is
auto-detected from byte 1 of the file: the font name is ASCII stored as
UTF-16LE, so the high byte of its first character is 0x00 in plaintext
and the raw byte equals the key. This reads every font in a GravoStyle
install regardless of version, not just one hardcoded key.

UI: right-clicking a text item in EntityView raises TextConvertRequested;
CadConverterForm renders it via ChrFont with H/V alignment and adds the
result on an ENGRAVE layer.

Tests use Xunit.SkippableFact and a gitignored test-config.json so the
suite points at a local .CHR file without committing proprietary assets.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

OpenNest.Tests/TestHelpers.cs

@@ -1,8 +1,37 @@
+using System.Text.Json;
 using OpenNest.CNC;
 using OpenNest.Geometry;
 
 namespace OpenNest.Tests;
 
+internal static class TestConfig
+{
+    private static readonly Lazy<Dictionary<string, string>> Config = new(() =>
+    {
+        var dir = AppContext.BaseDirectory;
+        for (var i = 0; i < 6; i++)
+        {
+            var path = Path.Combine(dir, "test-config.json");
+            if (File.Exists(path))
+            {
+                var json = File.ReadAllText(path);
+                return JsonSerializer.Deserialize<Dictionary<string, string>>(json) ?? new();
+            }
+            dir = Path.GetDirectoryName(dir)!;
+        }
+        return new();
+    });
+
+    public static string? Get(string key) =>
+        Config.Value.TryGetValue(key, out var val) ? val : null;
+
+    public static string? GetExistingPath(string key)
+    {
+        var path = Get(key);
+        return path != null && File.Exists(path) ? path : null;
+    }
+}
+
 internal static class TestHelpers
 {
     public static Part MakePartAt(double x, double y, double size = 1)