feat: add explicit left/right orientation with swap button

Fixes mirrored slice exports by making orientation explicit:

- PhotoSlice.swapped flag persists a user left/right swap
- swap_left_right() mirrors [TL,TR,BR,BL] -> [TR,TL,BL,BR]
- normalize_top_left() honors the flag so squarize/reset_rotation
  no longer overwrite manual orientation
- Swap L/R toolbar button wired through canvas state + controller
- Color-code left (orange) vs right (cyan) corners and edges

scanslicer/canvas_render.py

@@ -28,14 +28,22 @@ class CanvasRenderMixin:
                 b = points[(i + 1) % 4]
                 if i == 0:
                     color = QColor("red")
+                elif i == 1:
+                    color = QColor("green")
+                elif i == 2:
+                    color = QColor("orange")
                 else:
                     color = QColor("blue") if s.locked else QColor("lightgreen")
                 painter.setPen(QPen(color, 3))
                 painter.drawLine(a, b)
 
-            painter.setPen(QPen(QColor("blue"), 2))
-            painter.setBrush(QColor(128, 128, 128, 170))
-            for p in points:
+            # Corners are color-coded: left = orange, right = cyan.
+            # Edge 0 (top, red) runs TL->TR, so corners 0 and 3 are the left
+            # side and corners 1 and 2 are the right side.
+            corner_colors = (QColor("orange"), QColor("cyan"), QColor("cyan"), QColor("orange"))
+            for p, color in zip(points, corner_colors):
+                painter.setPen(QPen(QColor("blue"), 2))
+                painter.setBrush(color)
                 painter.drawRect(QRectF(p.x() - 5, p.y() - 5, 10, 10))
 
             center = np.mean(s.bbox, axis=0)

scanslicer/canvas_state.py

@@ -192,6 +192,15 @@ class CanvasStateMixin:
         self.update()
         return True
 
+    # swap_active_slice_left_right: function definition for ScanSlicer behavior.
+    def swap_active_slice_left_right(self):
+        active_slice = self.get_active_slice()
+        if active_slice is None:
+            return False
+        active_slice.swap_left_right()
+        self.update()
+        return True
+
     # reset_active_slice_rotation: function definition for ScanSlicer behavior.
     def reset_active_slice_rotation(self):
         active_slice = self.get_active_slice()

scanslicer/controller_canvas_actions.py

@@ -26,6 +26,7 @@ class ScanSlicerCanvasActionsMixin:
             self.active_box_lock_button.setEnabled(False)
             self.active_box_squarize_button.setEnabled(False)
             self.active_box_reset_rotation_button.setEnabled(False)
+            self.active_box_swap_button.setEnabled(False)
             self.active_box_remove_button.setEnabled(False)
             self.active_box_lock_button.blockSignals(True)
             self.active_box_lock_button.setChecked(False)
@@ -38,6 +39,7 @@ class ScanSlicerCanvasActionsMixin:
         self.active_box_lock_button.setEnabled(True)
         self.active_box_squarize_button.setEnabled(True)
         self.active_box_reset_rotation_button.setEnabled(True)
+        self.active_box_swap_button.setEnabled(True)
         self.active_box_remove_button.setEnabled(True)
         self.active_box_lock_button.blockSignals(True)
         self.active_box_lock_button.setChecked(active_slice.locked)
@@ -69,6 +71,13 @@ class ScanSlicerCanvasActionsMixin:
             return
         self.update_statusbar("Active box squarized.")
 
+    # swap_active_box_left_right: function definition for ScanSlicer behavior.
+    def swap_active_box_left_right(self):
+        if not self.slicing_canvas.swap_active_slice_left_right():
+            self.update_statusbar("No active box selected.")
+            return
+        self.update_statusbar("Active box left/right swapped.")
+
     # reset_active_box_rotation: function definition for ScanSlicer behavior.
     def reset_active_box_rotation(self):
         if not self.slicing_canvas.reset_active_slice_rotation():

scanslicer/main.py

@@ -230,6 +230,14 @@ class QtScanSlicer(QMainWindow, ScanSlicerUiBuilderMixin, ScanSlicerControllerMi
         "NzRMMjEgMTYiLz48cGF0aCBkPSJNMTYgMTZoNXY1Ii8+PGNpcmNsZSBjeD0iMTIiIGN5PSIxMiIgcj0iMSIv"
         "Pjwvc3ZnPg=="
     )
+    SWAP_LR_ICON_DATA_URL = (
+        "data:image/svg+xml;base64,"
+        "PHN2ZyB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmciIHdpZHRoPSIyNCIgaGVpZ2h0PSIyNCIg"
+        "dmlld0JveD0iMCAwIDI0IDI0IiBmaWxsPSJub25lIiBzdHJva2U9ImN1cnJlbnRDb2xvciIgc3Ryb2tlLXdp"
+        "ZHRoPSIyIiBzdHJva2UtbGluZWNhcD0icm91bmQiIHN0cm9rZS1saW5lam9pbj0icm91bmQiPjxwYXRoIGQ9"
+        "Ik04IDMgNCA3bDQgNCIvPjxwYXRoIGQ9Ik00IDdoMTYiLz48cGF0aCBkPSJtMTYgMjEgNC00LTQtNCIvPjxw"
+        "YXRoIGQ9Ik0yMCAxN0g0Ii8+PC9zdmc+"
+    )
     RESET_ROTATION_ICON_DATA_URL = (
         "data:image/svg+xml;base64,"
         "PHN2ZyB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmciIHdpZHRoPSIyNCIgaGVpZ2h0PSIyNCIg"

scanslicer/slices.py

@@ -18,6 +18,9 @@ class PhotoSlice:
         self.locked = False
         # Side-length handles are an explicit edit mode, enabled by squarize.
         self.side_handles_enabled = False
+        # False = default winding, True = explicit left/right swap by the user.
+        # Persists through squarize/reset_rotation so manual orientation wins.
+        self.swapped = False
 
     @property
     # bbox: function definition for ScanSlicer behavior.
@@ -49,8 +52,23 @@ class PhotoSlice:
         if horizontal_score(reversed_winding) > horizontal_score(ordered):
             ordered = reversed_winding
 
+        # A user-requested left/right swap overrides the geometric inference,
+        # so manual orientation survives normalize_top_left().
+        if getattr(self, "swapped", False):
+            ordered = ordered[[1, 0, 3, 2]]
+
         self.bbox = ordered
 
+    # swap_left_right: function definition for ScanSlicer behavior.
+    def swap_left_right(self):
+        # Mirror left/right: swap the two top corners and the two bottom
+        # corners, keeping the top (edge 0->1) and bottom edges in place.
+        # [TL, TR, BR, BL] <-> [TR, TL, BL, BR].
+        self.swapped = not self.swapped
+        self.bbox = self.bbox[[1, 0, 3, 2]]
+        self._rect_cache = None
+        self._rect_cache_tol = None
+
     # toggle_locked: function definition for ScanSlicer behavior.
     def toggle_locked(self, locked=None):
         if locked is not None:

scanslicer/ui_builders_preview.py

@@ -123,6 +123,16 @@ class ScanSlicerUiPreviewMixin(ScanSlicerUiSharedMixin):
         )
         preview_toolbar_layout.addWidget(self.active_box_reset_rotation_button)
 
+        self.active_box_swap_button = self._create_toolbar_button(
+            preview_toolbar,
+            text="Swap L/R",
+            tooltip="Swap left/right orientation of the active box",
+            icon_data_url=self.SWAP_LR_ICON_DATA_URL,
+            callback=self.swap_active_box_left_right,
+            enabled=False,
+        )
+        preview_toolbar_layout.addWidget(self.active_box_swap_button)
+
         self.add_box_button = self._create_toolbar_button(
             preview_toolbar,
             text="Add box",