feat(map): add batch export and adjust layout alignment

- Implement synchronous offscreen wafer map PNG rendering and CSV summary.
- Add "Batch Export" button to file browser sidebar.
- Adjust trend pane bottom spacer to align with About button when visible.
- Fix type check errors and add pytest coverage for batch export.

tests/test_wafer_map_item.py

@@ -8,20 +8,6 @@ import pytest
 pytestmark = pytest.mark.usefixtures("qapp")
 
 
-def test_export_image_valid_path():
-    """export_image saves a PNG when grabToImage succeeds."""
-    from pygui.backend.visualization.wafer_map_item import WaferMapItem
-
-    item = WaferMapItem()
-
-    mock_image = MagicMock()
-    mock_result = MagicMock()
-    mock_result.image.return_value = mock_image
-    item.grabToImage = MagicMock(return_value=mock_result)
-
-    assert item.export_image("/tmp/test_wafer.png") is True
-
-
 def test_export_image_empty_path():
     """export_image returns False for empty/None path."""
     from pygui.backend.visualization.wafer_map_item import WaferMapItem
@@ -31,29 +17,23 @@ def test_export_image_empty_path():
     assert item.export_image(None) is False
 
 
-def test_export_image_grab_fails():
-    """export_image returns False when grabToImage raises."""
+def test_export_image_zero_size_item_fails():
+    """export_image returns False when the item has no geometry to render."""
     from pygui.backend.visualization.wafer_map_item import WaferMapItem
 
     item = WaferMapItem()
-    item.grabToImage = MagicMock(side_effect=RuntimeError("grab failed"))
-
     assert item.export_image("/tmp/test_wafer.png") is False
 
 
-def test_export_image_save_fails():
-    """export_image returns False when image.save raises."""
+def test_export_image_unwritable_path_fails(tmp_path):
+    """export_image returns False when the target directory doesn't exist."""
     from pygui.backend.visualization.wafer_map_item import WaferMapItem
 
-    mock_image = MagicMock()
-    mock_image.save.side_effect = IOError("disk full")
-    mock_result = MagicMock()
-    mock_result.image.return_value = mock_image
-
     item = WaferMapItem()
-    item.grabToImage = MagicMock(return_value=mock_result)
-
-    assert item.export_image("/tmp/test_wafer.png") is False
+    item.setWidth(100)
+    item.setHeight(100)
+    item.values = [10.0, 20.0, 30.0]
+    assert item.export_image(str(tmp_path / "no_such_dir" / "wafer.png")) is False
 
 
 def test_thickness_properties():
@@ -88,3 +68,57 @@ def test_show_thickness_setter():
 
     item.showThickness = True
     assert item._show_thickness is True
+
+
+def test_export_image_writes_real_png_headless(tmp_path):
+    """export_image must produce an actual decodable image file without a
+    QQuickWindow — the button was silently failing because grabToImage()
+    resolves asynchronously and never completes headless/immediately."""
+    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]
+    out = tmp_path / "wafer.png"
+
+    assert item.export_image(str(out)) is True
+    assert out.exists()
+    loaded = QImage(str(out))
+    assert not loaded.isNull()
+    assert loaded.width() == 200
+
+
+def test_export_image_includes_metrics_footer(tmp_path):
+    """With sensor values loaded, the export gains a footer strip below the
+    map carrying the metric readouts (sensors / min / max / avg)."""
+    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]
+    out = tmp_path / "wafer.png"
+
+    assert item.export_image(str(out)) is True
+    loaded = QImage(str(out))
+    assert loaded.height() > 200  # footer strip appended
+    # footer is painted opaque (dark bg), not transparent
+    assert loaded.pixelColor(5, loaded.height() - 5).alpha() == 255
+
+
+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
+
+    item = WaferMapItem()
+    item.setWidth(200)
+    item.setHeight(200)
+    out = tmp_path / "wafer.png"
+
+    assert item.export_image(str(out)) is False
+    assert not out.exists()