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_perception.py

@@ -164,3 +164,88 @@ def test_tracker_reset_clears_state():
     t.reset()
     assert t.n_active() == 0
     assert t.step == 0
+
+
+# ---------------------------------------------------------------------------
+# Consensus promotion
+# ---------------------------------------------------------------------------
+
+def _tracker_with_consensus(k: int = 3, radius: float = 0.5, max_age: int = 8):
+    from herding.config import TrackerConfig
+    return SheepTracker(tracker_cfg=TrackerConfig(
+        consensus_k=k, consensus_radius_m=radius, consensus_max_age=max_age,
+    ))
+
+
+def test_consensus_default_disabled():
+    """With consensus_k=1 (default) the first detection is immediately visible."""
+    t = SheepTracker()
+    t.update([(5.0, 0.0)])
+    assert t.n_active() == 1
+    assert len(t.get_positions()) == 1
+
+
+def test_consensus_hides_one_shot_detection():
+    """K>=2: a single detection that never reappears is filtered out."""
+    t = _tracker_with_consensus(k=3)
+    t.update([(5.0, 0.0)])
+    assert t.n_active() == 0           # candidate, not promoted
+    assert t.n_candidate() == 1
+    assert t.get_positions() == {}
+
+
+def test_consensus_promotes_after_k_matches():
+    """A real sheep visible for K frames promotes and appears in get_positions."""
+    t = _tracker_with_consensus(k=3)
+    for _ in range(3):
+        t.update([(5.0, 0.0)])
+    assert t.n_active() == 1
+    assert t.n_candidate() == 0
+    assert len(t.get_positions()) == 1
+
+
+def test_consensus_candidate_expires_quickly():
+    """A candidate that fails to re-confirm within consensus_max_age dies."""
+    t = _tracker_with_consensus(k=3, max_age=5)
+    t.update([(5.0, 0.0)])
+    assert t.n_candidate() == 1
+    for _ in range(6):                  # > max_age empty frames
+        t.update([])
+    assert t.n_candidate() == 0
+    assert t.n_active() == 0
+
+
+def test_consensus_tracker_does_not_promote_phantom_pen():
+    """A one-shot detection inside the pen column must not latch as penned
+    while it is still a candidate."""
+    t = _tracker_with_consensus(k=3)
+    t.update([(11.5, -16.0)])           # gate-area FP, inside the pen column
+    # Not promoted, not penned — just a candidate.
+    assert t.n_penned() == 0
+    assert t.n_candidate() == 1
+    # And after one expiry window it disappears entirely.
+    for _ in range(10):
+        t.update([])
+    assert t.n_penned() == 0
+    assert t.n_candidate() == 0
+
+
+def test_consensus_distinguishes_real_sheep_from_phantom():
+    """Real sheep (continuous detections) promote; phantom (intermittent
+    detections at jittered positions outside consensus_radius) does not
+    appear in get_positions even while individual candidates are still
+    within the max-age window."""
+    t = _tracker_with_consensus(k=3, radius=0.4, max_age=4)
+    # Real sheep visible at (5, 0) every frame; phantom jitters > radius.
+    phantom_positions = [(10.0, 5.0), (10.5, 5.6), (11.1, 5.0), (10.0, 5.7)]
+    for k in range(4):
+        t.update([(5.0, 0.0), phantom_positions[k]])
+    positions = t.get_positions()
+    assert len(positions) == 1
+    real_xy = next(iter(positions.values()))
+    assert math.hypot(real_xy[0] - 5.0, real_xy[1]) < 0.5
+    # And once the candidate window has elapsed, every phantom has died.
+    for _ in range(8):
+        t.update([(5.0, 0.0)])
+    assert t.n_candidate() == 0
+    assert len(t.get_positions()) == 1