Phase 3: screenshot annotation with bounding boxes and @wN labels

- Add imageproc and ab_glyph dependencies
- Annotation module drawing colored bounding boxes per window
- White @wN labels on dark background at each window's top-left
- 8-color palette cycling for distinct window identification
- Back-to-front iteration so topmost labels are not occluded
- Embedded DejaVu Sans Mono font via include_bytes
- Wire --annotate flag into snapshot pipeline

src/backend/annotate.rs

@@ -0,0 +1,72 @@
+use ab_glyph::{FontArc, PxScale};
+use image::{Rgba, RgbaImage};
+use imageproc::drawing::{draw_filled_rect_mut, draw_hollow_rect_mut, draw_text_mut, text_size};
+use imageproc::rect::Rect;
+
+use crate::core::types::WindowInfo;
+
+// Embedded font - DejaVu Sans Mono for guaranteed availability
+const FONT_BYTES: &[u8] = include_bytes!("../../assets/DejaVuSansMono.ttf");
+
+const COLORS: &[Rgba<u8>] = &[
+    Rgba([0, 255, 0, 255]),     // green
+    Rgba([255, 100, 0, 255]),   // orange
+    Rgba([0, 150, 255, 255]),   // blue
+    Rgba([255, 0, 255, 255]),   // magenta
+    Rgba([255, 255, 0, 255]),   // yellow
+    Rgba([0, 255, 255, 255]),   // cyan
+    Rgba([255, 50, 50, 255]),   // red
+    Rgba([150, 100, 255, 255]), // purple
+];
+
+const LABEL_BG: Rgba<u8> = Rgba([0, 0, 0, 200]);
+const LABEL_FG: Rgba<u8> = Rgba([255, 255, 255, 255]);
+
+pub fn annotate_screenshot(image: &mut RgbaImage, windows: &[WindowInfo]) {
+    let font = match FontArc::try_from_slice(FONT_BYTES) {
+        Ok(f) => f,
+        Err(_) => return, // Silently skip annotation if font fails to load
+    };
+    let scale = PxScale { x: 18.0, y: 18.0 };
+
+    // Iterate back-to-front so topmost window labels are drawn last (not occluded)
+    for (i, win) in windows.iter().enumerate().rev() {
+        if win.minimized {
+            continue; // Don't annotate hidden windows
+        }
+
+        let color = COLORS[i % COLORS.len()];
+        let x = win.x.max(0);
+        let y = win.y.max(0);
+        let w = win.width;
+        let h = win.height;
+
+        // Draw bounding box (2px thick by drawing at offset 0 and 1)
+        if w > 0 && h > 0 {
+            draw_hollow_rect_mut(image, Rect::at(x, y).of_size(w, h), color);
+            if w > 2 && h > 2 {
+                draw_hollow_rect_mut(
+                    image,
+                    Rect::at(x + 1, y + 1).of_size(w - 2, h - 2),
+                    color,
+                );
+            }
+        }
+
+        // Draw label
+        let label = format!("@{}", win.ref_id);
+        let (tw, th) = text_size(scale, &font, &label);
+        let label_x = x;
+        let label_y = (y - th as i32 - 4).max(0);
+
+        // Label background
+        draw_filled_rect_mut(
+            image,
+            Rect::at(label_x, label_y).of_size(tw + 6, th + 4),
+            LABEL_BG,
+        );
+
+        // Label text
+        draw_text_mut(image, LABEL_FG, label_x + 3, label_y + 2, scale, &font, &label);
+    }
+}