feat(map): add peak highlights and live metrics to PNG export

- Draw rings around hottest and coldest sensors on export.
- Extend export_image to support second line for live metrics.
- Format detailed sensor stats in export footer.
- Add test coverage for footer rows, overlays, and stats.

tests/test_wafer_map_item.py

@@ -111,6 +111,76 @@ def test_export_image_includes_metrics_footer(tmp_path):
     assert loaded.pixelColor(5, loaded.height() - 5).alpha() == 255
 
 
+def test_export_image_extra_line_grows_footer(tmp_path):
+    """Live export passes an extra metrics line; the footer gains a second
+    row for it. Review export (no extra) keeps the single-row footer."""
+    from PySide6.QtGui import QImage
+
+    from pygui.backend.visualization.wafer_map_item import WaferMapItem
+
+    item = WaferMapItem()
+    item.setWidth(200)
+    item.setHeight(200)
+    item.values = [10.0, 20.0, 30.0]
+
+    review = tmp_path / "review.png"
+    live = tmp_path / "live.png"
+    assert item.export_image(str(review)) is True
+    assert item.export_image(str(live), "Frames: 42   Errors: 0") is True
+
+    h_review = QImage(str(review)).height()
+    h_live = QImage(str(live)).height()
+    assert h_review > 200  # readout footer present
+    assert h_live > h_review  # extra live row appended
+
+
+def test_export_image_highlights_hottest_and_coldest(tmp_path):
+    """The exported map rings the hottest sensor in the high color and the
+    coldest in the low color. With no bands set every marker fill is the
+    in-range green, so any high/low-colored pixels must come from the rings."""
+    from PySide6.QtGui import QImage
+
+    from pygui.backend.visualization.wafer_map_item import WaferMapItem
+
+    item = WaferMapItem()
+    item.setWidth(200)
+    item.setHeight(200)
+    item.sensors = [
+        {"label": "1", "x": -50.0, "y": 0.0},
+        {"label": "2", "x": 0.0, "y": 0.0},
+        {"label": "3", "x": 50.0, "y": 0.0},
+    ]
+    item.values = [10.0, 20.0, 30.0]
+    out = tmp_path / "wafer.png"
+    assert item.export_image(str(out)) is True
+
+    img = QImage(str(out))
+    found = {img.pixelColor(x, y).name()
+             for y in range(img.height()) for x in range(img.width())}
+    assert "#ef4444" in found  # hottest ring (sensorHigh)
+    assert "#5b9df5" in found  # coldest ring (sensorLow)
+
+
+def test_readout_text_full_stats():
+    """Export footer readout carries every READOUT stat with sensor indices.
+
+    Worked example: [10, 20, 30] → min 10 (#1), max 30 (#3), diff 20,
+    avg 20, σ = sqrt(200/3) ≈ 8.16, 3σ ≈ 24.49.
+    """
+    from pygui.backend.visualization.wafer_map_item import readout_text
+
+    assert readout_text([10.0, 20.0, 30.0]) == (
+        "Sensors: 3   Min: 10.00°C (#1)   Max: 30.00°C (#3)   "
+        "Diff: 20.00   Avg: 20.00   σ: 8.16   3σ: 24.49"
+    )
+
+
+def test_readout_text_empty():
+    from pygui.backend.visualization.wafer_map_item import readout_text
+
+    assert readout_text([]) == ""
+
+
 def test_export_image_no_data_fails(tmp_path):
     """No file loaded / no stream played -> nothing meaningful to export."""
     from pygui.backend.visualization.wafer_map_item import WaferMapItem