feat: stabilize secondary-model detections, box-wins suppression, port 9050

- SyntheticIdAssigner: track_id=None dets get stable negative IDs so the
  stabilizer hold bridges sup_det_interval gaps -> no flicker, boxes count
- apply primary model class filter in merged path (UI checkboxes work)
- _suppress_sacks_under_boxes: sack dets overlapping box dets dropped
  (models without box class label boxes as sack; stops double counting)
- default web port 9000 -> 9050 (WEB_PORT env still overrides)
- README: port refs, counting zone modes section, project tree
- 140 tests pass (+13)

tests/test_synthetic_track.py

@@ -0,0 +1,133 @@
+"""Tests for synthetic track-ID assignment (src/synthetic_track.py)
+and sack-under-box suppression (src.pipeline)."""
+
+import pytest
+
+from src.interfaces import Detection
+from src.pipeline import _suppress_sacks_under_boxes
+from src.synthetic_track import SyntheticIdAssigner
+
+
+def det(bbox, class_name="sack", track_id=None, conf=0.9):
+    return Detection(
+        bbox=tuple(bbox), confidence=conf, class_id=0,
+        class_name=class_name, track_id=track_id,
+    )
+
+
+# ── SyntheticIdAssigner ─────────────────────────────────────────────────
+
+
+def test_assigns_new_negative_id_to_untracked_det():
+    """track_id=None det gets a fresh negative id (ByteTrack uses positive)."""
+    a = SyntheticIdAssigner()
+    out = a.assign([det((0, 0, 10, 10))])
+    assert out[0].track_id is not None
+    assert out[0].track_id < 0
+    assert out[0].track_id == -1
+
+
+def test_same_position_reuses_id():
+    """Same det next call keeps its id (registry bbox within IoU 0.3)."""
+    a = SyntheticIdAssigner()
+    first = a.assign([det((0, 0, 10, 10))])[0].track_id
+    second = a.assign([det((1, 1, 11, 11))])[0].track_id  # 5px jitter
+    assert second == first
+
+
+def test_moved_beyond_iou_gets_new_id():
+    """Det teleports beyond IoU threshold → new id, old entry retained."""
+    a = SyntheticIdAssigner()
+    first = a.assign([det((0, 0, 10, 10))])[0].track_id
+    moved = a.assign([det((100, 100, 110, 110))])[0].track_id
+    assert moved != first
+    assert moved == first - 1
+
+
+def test_class_mismatch_never_shares_id():
+    """box next to sack must not adopt the sack's id."""
+    a = SyntheticIdAssigner()
+    sack_id = a.assign([det((0, 0, 10, 10), class_name="sack")])[0].track_id
+    box_id = a.assign([det((0, 0, 10, 10), class_name="box")])[0].track_id
+    assert box_id != sack_id
+
+
+def test_real_track_id_passes_through_untouched():
+    """ByteTrack det keeps its positive id; registry stays empty."""
+    a = SyntheticIdAssigner()
+    out = a.assign([det((0, 0, 10, 10), track_id=7)])
+    assert out[0].track_id == 7
+    assert a._registry == {}
+
+
+def test_expiry_boundaries():
+    """Entry expires after max_age (10) consecutive unmatched calls.
+
+    Note: spec bullet said "10 calls → gone"; literal `age > max_age`
+    (matches BboxStabilizer's `age > max_hold`) expires on the 11th miss.
+    9 and 10 unmatched calls both keep the entry alive.
+    """
+    a = SyntheticIdAssigner()  # max_age=10
+    a.assign([det((0, 0, 10, 10))])
+    assert -1 in a._registry
+
+    for _ in range(10):
+        a.assign([])
+    assert -1 in a._registry  # still held at age 10
+
+    a.assign([])
+    assert -1 not in a._registry  # gone at age 11
+
+
+def test_fresh_det_after_expiry_gets_new_id():
+    """Old id is not reused once the entry expired."""
+    a = SyntheticIdAssigner()
+    first = a.assign([det((0, 0, 10, 10))])[0].track_id
+    for _ in range(11):
+        a.assign([])
+    again = a.assign([det((0, 0, 10, 10))])[0].track_id
+    assert again != first
+
+
+def test_clear_empties_registry():
+    a = SyntheticIdAssigner()
+    a.assign([det((0, 0, 10, 10)), det((50, 50, 60, 60))])
+    assert a._registry
+    a.clear()
+    assert a._registry == {}
+
+
+# ── _suppress_sacks_under_boxes ─────────────────────────────────────────
+
+
+def test_overlapping_sack_dropped_box_kept():
+    """IoU > 0.5 between sack and box → sack dropped, box wins."""
+    box = det((0, 0, 100, 100), class_name="box")
+    sack = det((10, 10, 90, 90), class_name="sack")  # IoU = 0.64
+    out = _suppress_sacks_under_boxes([box, sack])
+    assert [d.class_name for d in out] == ["box"]
+
+
+def test_no_box_detections_unchanged():
+    dets = [det((0, 0, 10, 10)), det((50, 50, 60, 60))]
+    assert _suppress_sacks_under_boxes(dets) == dets
+
+
+def test_sack_far_from_box_kept():
+    box = det((0, 0, 100, 100), class_name="box")
+    far = det((200, 200, 250, 250), class_name="sack")  # IoU = 0
+    out = _suppress_sacks_under_boxes([box, far])
+    assert len(out) == 2
+
+
+def test_two_sacks_unaffected():
+    """Two non-overlapping sacks stay when a box is present elsewhere."""
+    box = det((0, 0, 100, 100), class_name="box")
+    s1 = det((200, 0, 250, 50), class_name="sack")
+    s2 = det((200, 200, 250, 250), class_name="sack")
+    out = _suppress_sacks_under_boxes([box, s1, s2])
+    assert out == [box, s1, s2]
+
+
+def test_empty_input():
+    assert _suppress_sacks_under_boxes([]) == []