feat: initial release of SplitOverlay (KeyOverlay 1.0.0)

Semi-transparent always-on-top Windows overlay that shows the keymap of a
Vial/QMK split keyboard (Silakka54).

- draws all layers from a Vial .vil layout, transparent keys fall through
- detects the active layer from the keyboard via a Raw HID command (firmware/)
- shows characters of the active Windows input language
- can split the two halves into separate, independently movable windows
- highlights pressed keys; tray menu for opacity, size and options
- Inno Setup installer and portable build; GitHub-style CI workflow

tests/KeyOverlay.Core.Tests/KeyOverlay.Core.Tests.csproj

@@ -0,0 +1,16 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <TargetFramework>net8.0</TargetFramework>
+    <Nullable>enable</Nullable>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
+  <ItemGroup>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.11.1" />
+    <PackageReference Include="xunit" Version="2.9.2" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="2.8.2" />
+  </ItemGroup>
+  <ItemGroup>
+    <ProjectReference Include="..\..\src\KeyOverlay.Core\KeyOverlay.Core.csproj" />
+  </ItemGroup>
+</Project>