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/Dockerfile

@@ -0,0 +1,6 @@
+# Build image for the Vial firmware: QMK base container + ARM toolchain + qmk CLI.
+FROM ghcr.io/qmk/qmk_base_container:latest
+RUN apt-get update \
+ && apt-get install -y --no-install-recommends gcc-arm-none-eabi binutils-arm-none-eabi libnewlib-arm-none-eabi \
+ && rm -rf /var/lib/apt/lists/* \
+ && pip3 install --break-system-packages --root-user-action=ignore -q qmk