Add redo, undo×10, brush preview, hide/show mask; drop Reload Saved

mask_canvas.py:
- Redo stack: new strokes clear it, undo pushes onto it, redo pops from it.
- undo10(): undoes up to 10 steps in one call with a single redraw.
- Brush circle preview: white Circle patch tracks mouse position and shows
  current brush radius; hidden when cursor leaves the axes.
- toggle_mask() / btn_mask: hides or shows the green mask overlay without
  affecting the underlying mask data.

annotator.py:
- Removed Reload Saved button and reload_saved() — clip already loads its
  saved state on navigation, making the button redundant.
- Added Undo×10 and Redo buttons wired to mc.undo10 / mc.redo.
- Added Hide Mask button (mc.btn_mask) to the toolbar row.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/river_annotation_tool/annotator.py

@@ -111,14 +111,22 @@ class Annotator(QMainWindow):
         btn_skip = QPushButton("Skip")
         btn_clear = QPushButton("Clear")
         btn_undo = QPushButton("Undo")
-        btn_reload = QPushButton("Reload Saved")
+        btn_undo10 = QPushButton("Undo×10")
+        btn_redo = QPushButton("Redo")
 
         row1 = QHBoxLayout()
         for b in [self.btn_prev, btn_next, btn_skip]:
             row1.addWidget(b)
 
         row2 = QHBoxLayout()
-        for b in [btn_clear, self.mc.btn_erase, btn_undo, btn_reload]:
+        for b in [
+            btn_clear,
+            self.mc.btn_erase,
+            btn_undo,
+            btn_undo10,
+            btn_redo,
+            self.mc.btn_mask,
+        ]:
             row2.addWidget(b)
         row2.addWidget(QLabel("Brush"))
         row2.addWidget(self.mc.brush_slider)
@@ -146,7 +154,8 @@ class Annotator(QMainWindow):
         btn_skip.clicked.connect(self.skip_clip)
         btn_clear.clicked.connect(self.mc.clear)
         btn_undo.clicked.connect(self.mc.undo)
-        btn_reload.clicked.connect(self.reload_saved)
+        btn_undo10.clicked.connect(self.mc.undo10)
+        btn_redo.clicked.connect(self.mc.redo)
 
         if self._pending_answers:
             self._set_answers(self._pending_answers)
@@ -258,15 +267,6 @@ class Annotator(QMainWindow):
 
         print("Saved:", out)
 
-    def reload_saved(self):
-        mask = self._read_saved_mask()
-        if mask is None:
-            return
-        self.mc.reset(mask)
-        answers = self._read_saved_answers()
-        if answers:
-            self._set_answers(answers)
-
     def _switch_ui_to_clip(self):
         self.frame_i = 0
         self.mc.load_clip(