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/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: