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

Cargo.toml

@@ -17,3 +17,5 @@ libc = "0.2"
 uuid = { version = "1", features = ["v4"] }
 xcap = "0.8"
 image = { version = "0.25", features = ["png"] }
+imageproc = "0.26"
+ab_glyph = "0.2"