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/OpenNest.Tests.csproj

@@ -14,6 +14,7 @@
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.8.0" />
     <PackageReference Include="xunit" Version="2.5.3" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.5.3" />
+    <PackageReference Include="Xunit.SkippableFact" Version="1.4.13" />
   </ItemGroup>
 
   <ItemGroup>
@@ -38,6 +39,9 @@
     <Content Include="Splitting\TestData\**\*">
       <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
     </Content>
+    <Content Include="test-config.json" Condition="Exists('test-config.json')">
+      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+    </Content>
   </ItemGroup>
 
 </Project>