fix(vision): erode mask pre-CC to sever anti-aliasing bridges

With tol=25 all dots in the strip fused into one blob via 1px anti-aliased
bridges between adjacent dots → centroid landed mid-strip instead of on the
rightmost dot. Erosion (3x3 kernel, 2 iters) cleanly separates discrete
dots before connected-components labelling.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/atm/vision.py

@@ -103,9 +103,11 @@ def find_rightmost_dot(
     diff = np.linalg.norm(roi_img.astype(np.float32) - bgr_bg, axis=2)
     mask = (diff > bg_tol).astype(np.uint8)
 
-    # Connected components → one component per dot. Anti-aliasing bridges
-    # between adjacent dots are small enough that 8-connectivity still
-    # separates them cleanly.
+    # Anti-aliasing bridges between adjacent dots can fuse the entire strip
+    # into one blob under 8-connectivity. Erode by 2px to sever those bridges
+    # before labelling — dots shrink but remain well above min_cluster_px.
+    kernel = np.ones((3, 3), dtype=np.uint8)
+    mask = cv2.erode(mask, kernel, iterations=2)
     n_labels, _labels, stats, centroids = cv2.connectedComponentsWithStats(
         mask, connectivity=8,
     )