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

firmware/keyoverlay_hid.c

@@ -0,0 +1,31 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+//
+// KeyOverlay: lets the host ask which layer is active.
+//
+// Request  (host -> keyboard): [0xE0, 0x01, ...]
+// Response (keyboard -> host): [0xE0, 0x01, highest_layer, layer_state (u32 LE), default_layer_state (u32 LE), ...]
+//
+// 0xE0 is outside the ids used by VIA (0x01..0x1F), Vial (0xFE) and "unhandled" (0xFF).
+// Only reads state; it changes nothing on the keyboard and works while Vial is locked.
+#include QMK_KEYBOARD_H
+#include "raw_hid.h"
+#include <string.h>
+
+#define KEYOVERLAY_CMD 0xE0
+#define KEYOVERLAY_PROTOCOL 0x01
+#define KEYOVERLAY_ID_UNHANDLED 0xFF
+#define KEYOVERLAY_MIN_LENGTH 11
+
+void raw_hid_receive_kb(uint8_t *data, uint8_t length) {
+    if (data[0] != KEYOVERLAY_CMD || length < KEYOVERLAY_MIN_LENGTH) {
+        data[0] = KEYOVERLAY_ID_UNHANDLED;
+        return;
+    }
+    const uint32_t active  = (uint32_t)layer_state;
+    const uint32_t default_ = (uint32_t)default_layer_state;
+
+    data[1] = KEYOVERLAY_PROTOCOL;
+    data[2] = get_highest_layer(layer_state | default_layer_state);
+    memcpy(&data[3], &active, sizeof(active));
+    memcpy(&data[7], &default_, sizeof(default_));
+}