Consensus tracker + active scan close Webots 140° LiDAR gap

Two deploy-time fixes that take v1 360°-trained BC/RL from 0/n to n/n
penned on the canonical 140° LiDAR proto for diff/field:

* SheepTracker now supports a consensus stage: new detections start as
  candidate tracks invisible to get_positions(). A candidate must
  accumulate consensus_k matches within consensus_radius_m of itself
  inside a consensus_max_age window to be promoted; otherwise it
  expires. Real sheep self-confirm within 3 frames (≪0.05 m/step);
  wall-return cluster centroids jitter beyond 0.3 m as the dog moves
  and never promote. consensus_k=1 (default) is a no-op so unconfigured
  callers and HERDING_DEFAULT keep prior behaviour.
* HERDING_WEBOTS preset gets consensus_k=3, radius=0.3, max_age=20,
  plus longer forget_steps=300 and predict_steps=180 so confirmed
  sheep persist through long FOV-occlusion gaps a narrow 140° cone
  produces. max_new_tracks_per_step=1 still rate-caps spawn bursts.
* shepherd_dog.py BC/RL empty-obs fallback now rotates the desired
  heading with step_count so the cone actively sweeps the field
  instead of driving due north into the wall.

Verified in headless Webots (HERDING_USE_GT=0, LiDAR only):
  BC diff/field:        5/5 @ 11698, 10/10 @ 15079
  RL diff/field:        5/5 @ 10039, 9/10 @ 18200 (timeout)
  Strömbom diff/field:  5/5 @ 7528
All previously 0/n. 120 unit tests pass; 9 new consensus tests cover
the candidate stage, promotion radius, and one-shot phantom rejection.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tests/test_config.py

@@ -61,10 +61,23 @@ class TestTrackerConfig:
 
     def test_webots_preset_tighter(self):
         cfg = HERDING_WEBOTS.tracker
-        assert cfg.forget_steps == 120
+        # forget_steps was extended so confirmed sheep tracks survive
+        # sparse 140° FOV re-sightings; consensus blocks phantoms from
+        # reaching this lifetime.
+        assert cfg.forget_steps >= 200
         assert cfg.max_new_tracks_per_step == 1
         assert cfg.pen_latch_depth == 2.0
 
+    def test_default_consensus_disabled(self):
+        cfg = TrackerConfig()
+        assert cfg.consensus_k == 1
+
+    def test_webots_preset_enables_consensus(self):
+        cfg = HERDING_WEBOTS.tracker
+        assert cfg.consensus_k > 1
+        assert cfg.consensus_radius_m > 0.0
+        assert cfg.consensus_max_age >= cfg.consensus_k
+
     def test_invalid_forget_steps(self):
         with pytest.raises(ValueError):
             TrackerConfig(forget_steps=0)
@@ -73,6 +86,14 @@ class TestTrackerConfig:
         with pytest.raises(ValueError):
             TrackerConfig(max_new_tracks_per_step=0)
 
+    def test_invalid_consensus_params(self):
+        with pytest.raises(ValueError):
+            TrackerConfig(consensus_k=0)
+        with pytest.raises(ValueError):
+            TrackerConfig(consensus_radius_m=0.0)
+        with pytest.raises(ValueError):
+            TrackerConfig(consensus_max_age=0)
+
 
 # ---------------------------------------------------------------------------
 # DetectionConfig