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/prepare-source.sh

@@ -0,0 +1,32 @@
+#!/usr/bin/env bash
+# Prepares the firmware source tree: vial-qmk + the Silakka54 LED keymap (v1.4led) + the KeyOverlay HID command.
+# Usage: bash firmware/prepare-source.sh [work-dir]     (default work-dir: .firmware-work next to this repo)
+# Safe to run again: sources are only cloned once, copied files are refreshed.
+set -euo pipefail
+
+HERE="$(cd "$(dirname "$0")" && pwd)"
+WORK="${1:-$HERE/../.firmware-work}"
+QMK="$WORK/vial-qmk"
+SRC="$WORK/silakka54-src"
+DEST="$QMK/keyboards/silakka54led"
+
+mkdir -p "$WORK"
+
+if [ ! -d "$QMK" ]; then
+  git clone --depth 1 --branch vial https://github.com/vial-kb/vial-qmk.git "$QMK"
+  git -C "$QMK" submodule update --init --depth 1 --recursive
+fi
+if [ ! -d "$SRC" ]; then
+  git clone --depth 1 --branch v1.4led https://github.com/Squalius-cephalus/silakka54.git "$SRC"
+fi
+
+# The LED keymap lives in the silakka54 repo, not in vial-qmk: copy it in as its own keyboard folder.
+mkdir -p "$DEST"
+cp -r "$SRC/firmware/." "$DEST/"
+
+cp "$HERE/keyoverlay_hid.c" "$DEST/keymaps/vial_led/"
+RULES="$DEST/keymaps/vial_led/rules.mk"
+grep -q "keyoverlay_hid.c" "$RULES" || printf '\nSRC += keyoverlay_hid.c\n' >> "$RULES"
+
+echo "Firmware source ready in: $QMK"
+echo "Next: docker build -t keyoverlay-fw-build firmware, then run firmware/build-in-container.sh (see firmware/README.md)"