Add link management: create, select, delete with undo/redo

Implement full link workflow between component slots on the wiresheet:
- Link Mark / Link From / Link To context menu items for creating links
- Link dialog with slot type validation and source/target swap button
- Selectable link wires (click to highlight in red) with right-click delete
- Undo/redo support for both add and delete link operations
- Component deletion already cascades to remove related links

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

undo.py

@@ -124,6 +124,41 @@ class AddComponentCommand:
             self.parent.insert(self.index, self.element)
 
 
+class AddLinkCommand:
+    """Adding a single link to the <links> section."""
+
+    def __init__(self, links_parent, link_element, label="add link"):
+        self.links_parent = links_parent
+        self.link_element = link_element
+        self.label = label
+
+    def undo(self):
+        if self.link_element in list(self.links_parent):
+            self.links_parent.remove(self.link_element)
+
+    def redo(self):
+        if self.link_element not in list(self.links_parent):
+            self.links_parent.append(self.link_element)
+
+
+class DeleteLinkCommand:
+    """Removing a single link from the <links> section."""
+
+    def __init__(self, links_parent, index, link_element, label="delete link"):
+        self.links_parent = links_parent
+        self.index = index
+        self.link_element = link_element
+        self.label = label
+
+    def undo(self):
+        if self.link_element not in list(self.links_parent):
+            self.links_parent.insert(self.index, self.link_element)
+
+    def redo(self):
+        if self.link_element in list(self.links_parent):
+            self.links_parent.remove(self.link_element)
+
+
 class DeleteComponentCommand:
     """A cascading delete: the component subtree plus every link that referenced it."""