feat: initial project with CPU-rendered ring light overlay

Wayland layer-shell overlay using tiny-skia for rendering a glowing
rounded rectangle ring. Includes PID-based lock file toggle via SIGUSR1,
keyboard capture (Escape to exit), and pointer tracking for cursor
avoidance. Configurable ring geometry and glow parameters.

src/main.rs

@@ -0,0 +1,74 @@
+mod lock;
+mod render;
+mod wayland;
+
+use std::time::{Duration, Instant};
+
+use anyhow::{Context, Result};
+use tiny_skia::Pixmap;
+
+use lock::ToggleResult;
+use wayland::{OverlayEvent, WaylandBackend};
+
+const DISPLAY_DURATION: Duration = Duration::from_secs(5);
+
+fn main() -> Result<()> {
+    match lock::try_toggle()? {
+        ToggleResult::SignaledExisting => return Ok(()),
+        ToggleResult::RunNew => {}
+    }
+
+    let _lock = lock::create_lock()?;
+
+    let mut wl = WaylandBackend::new()?;
+    let (w, h) = wl.screen_size();
+    let mut pixmap = Pixmap::new(w, h).context("create pixmap")?;
+    let mut argb_buf = vec![0u8; (w as usize) * (h as usize) * 4];
+
+    let mut cursor_x = w as f64 / 2.0;
+    let mut cursor_y = h as f64 / 2.0;
+
+    draw(&mut wl, &mut pixmap, &mut argb_buf, cursor_x, cursor_y)?;
+
+    let start = Instant::now();
+
+    loop {
+        let elapsed = start.elapsed();
+        if elapsed >= DISPLAY_DURATION || lock::should_exit() {
+            break;
+        }
+        let remaining = DISPLAY_DURATION - elapsed;
+        let event = match wl.next_event(remaining)? {
+            Some(ev) => ev,
+            None => continue, // poll timeout — loop back to check elapsed time
+        };
+        match event {
+            OverlayEvent::Close => break,
+            OverlayEvent::CursorMoved(x, y) => {
+                cursor_x = x;
+                cursor_y = y;
+                draw(&mut wl, &mut pixmap, &mut argb_buf, cursor_x, cursor_y)?;
+            }
+            OverlayEvent::Configure(new_w, new_h) => {
+                if new_w != w || new_h != h {
+                    break;
+                }
+            }
+        }
+    }
+
+    Ok(())
+}
+
+fn draw(
+    wl: &mut WaylandBackend,
+    pixmap: &mut Pixmap,
+    argb_buf: &mut [u8],
+    cursor_x: f64,
+    cursor_y: f64,
+) -> Result<()> {
+    render::render_ring_light(pixmap, cursor_x, cursor_y);
+    render::pixmap_to_argb8888(pixmap, argb_buf);
+    let (w, h) = wl.screen_size();
+    wl.present(argb_buf, w, h)
+}