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/LayerProtocolTests.cs

@@ -0,0 +1,65 @@
+using KeyOverlay.Core;
+using Xunit;
+
+namespace KeyOverlay.Core.Tests;
+
+public class LayerProtocolTests
+{
+    [Fact]
+    public void BuildRequest_Is32BytesWithCommandAndVersion()
+    {
+        var request = LayerProtocol.BuildRequest();
+        Assert.Equal(32, request.Length);
+        Assert.Equal(0xE0, request[0]);
+        Assert.Equal(0x01, request[1]);
+    }
+
+    [Fact]
+    public void TryParse_ReadsLayerAndMasks()
+    {
+        var payload = new byte[32];
+        payload[0] = 0xE0;
+        payload[1] = 0x01;
+        payload[2] = 2;
+        payload[3] = 0b0000_0101; // layer_state = layers 0 and 2
+        payload[7] = 0b0000_0001; // default_layer_state = layer 0
+
+        Assert.True(LayerProtocol.TryParse(payload, out var report));
+        Assert.Equal(2, report.HighestLayer);
+        Assert.Equal(5u, report.LayerState);
+        Assert.Equal(1u, report.DefaultLayerState);
+    }
+
+    [Fact]
+    public void TryParse_ReadsLittleEndianMask()
+    {
+        var payload = new byte[32];
+        payload[0] = 0xE0;
+        payload[1] = 0x01;
+        payload[4] = 0x01; // bit 8 of layer_state
+
+        Assert.True(LayerProtocol.TryParse(payload, out var report));
+        Assert.Equal(0x100u, report.LayerState);
+    }
+
+    [Fact]
+    public void TryParse_RejectsUnhandledResponse()
+    {
+        var payload = new byte[32];
+        payload[0] = 0xFF; // old firmware: "unhandled"
+        Assert.False(LayerProtocol.TryParse(payload, out _));
+    }
+
+    [Fact]
+    public void TryParse_RejectsUnknownProtocolVersion()
+    {
+        var payload = new byte[32];
+        payload[0] = 0xE0;
+        payload[1] = 0x7F;
+        Assert.False(LayerProtocol.TryParse(payload, out _));
+    }
+
+    [Fact]
+    public void TryParse_RejectsShortPayload() =>
+        Assert.False(LayerProtocol.TryParse(new byte[5], out _));
+}