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/build-in-container.sh

@@ -0,0 +1,18 @@
+#!/bin/sh
+# Runs inside the keyoverlay-fw-build image with vial-qmk mounted at /qmk_firmware.
+# Builds the silakka54 vial_led keymap twice: MASTER_LEFT (left half, USB side) and MASTER_RIGHT.
+set -e
+cd /qmk_firmware
+git config --global --add safe.directory '*'
+KEYMAP_CFG=keyboards/silakka54led/keymaps/vial_led/config.h
+sed -i "/MASTER_/d" "$KEYMAP_CFG"   # drop leftovers from an interrupted run
+cp "$KEYMAP_CFG" /tmp/config.h.orig
+
+for SIDE in LEFT RIGHT; do
+  cp /tmp/config.h.orig "$KEYMAP_CFG"
+  printf '\n#define MASTER_%s\n' "$SIDE" >> "$KEYMAP_CFG"
+  make silakka54led:vial_led
+  cp silakka54led_vial_led.uf2 "silakka54_vial_led_keyoverlay_$SIDE.uf2"
+done
+cp /tmp/config.h.orig "$KEYMAP_CFG"
+ls -la silakka54_vial_led_keyoverlay_*.uf2