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

.github/workflows/ci.yml

@@ -0,0 +1,19 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+jobs:
+  build-and-test:
+    runs-on: windows-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-dotnet@v4
+        with:
+          dotnet-version: 8.0.x
+      - name: Test core library
+        run: dotnet test tests/KeyOverlay.Core.Tests --configuration Release
+      - name: Build overlay app
+        run: dotnet build src/KeyOverlay --configuration Release