feat: CV line-crossing entry/exit detection with tests

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

firmware/lib/cv/cv.cpp

@@ -146,6 +146,23 @@ CVResult cv_process(CVState& state, const uint8_t* frame, uint8_t line_pct) {
         t.missed     = 0;
         state.tracks.push_back(t);
     }
-    // Line crossing check added in Task 6
+    // Line crossing check
+    for (auto& track : state.tracks) {
+        if (track.missed > 0) continue; // only check tracks matched this frame
+        bool now_above = (track.y < line_y);
+        if (now_above != track.above_line) {
+            if (!now_above) {
+                // was above, now below → entry
+                state.entries++;
+                result.entries_delta++;
+            } else {
+                // was below, now above → exit
+                state.exits++;
+                result.exits_delta++;
+            }
+        }
+        track.above_line = now_above;
+    }
+
     return result;
 }