add README

README.md

+# ximper-shell-osd
+
+On-screen display daemon.
+
+A component of the Ximper Shell.
+
+Displays volume and brightness OSD overlays on Wayland. Built with Vala, GTK4, libadwaita, gtk4-layer-shell.
+
+## Build
+
+```sh
+meson setup _build
+ninja -C _build
+./_build/src/ximper-shell-osd
+```
+
+### Build options
+
+| Option        | Default | Description                              |
+| ------------- | ------- | ---------------------------------------- |
+| `pulse-audio` | `true`  | Monitor PulseAudio volume changes        |
+| `ddc`         | `true`  | Monitor DDC/CI brightness via libddcutil |
+
+```sh
+meson setup _build -Dddc=false
+```
+
+## Config
+
+`~/.config/ximper-shell/osd/config.json`
+
+```json
+{
+  "timeout_ms": 1500,
+  "margin_bottom": 80,
+  "width": 300,
+  "monitor_volume": true,
+  "monitor_brightness": true,
+  "monitor_ddc": true,
+  "ddc_poll_interval_sec": 5
+}
+```
+
+| Key                     | Type | Default | Description                       |
+| ----------------------- | ---- | ------- | --------------------------------- |
+| `timeout_ms`            | int  | `1500`  | OSD display duration in ms        |
+| `margin_bottom`         | int  | `80`    | Bottom margin in pixels           |
+| `width`                 | int  | `300`   | OSD window width in pixels        |
+| `monitor_volume`        | bool | `true`  | Monitor PulseAudio volume changes |
+| `monitor_brightness`    | bool | `true`  | Monitor sysfs backlight changes   |
+| `monitor_ddc`           | bool | `true`  | Monitor DDC/CI brightness changes |
+| `ddc_poll_interval_sec` | int  | `5`     | DDC polling interval in seconds   |
+
+All fields are optional — defaults are used when config is absent.
+
+## D-Bus
+
+`ru.ximperlinux.shell.OSD` at `/ru/ximperlinux/shell/OSD`:
+- `ShowVolumeOsd(double percent, bool muted)`
+- `ShowBrightnessOsd(double percent)`
+
+## CSS
+
+Default style is installed to `/etc/xdg/ximper-shell/osd/style.css`.
+User override: `~/.config/ximper-shell/osd/style.css`.
+
+## License
+
+GPL-3.0