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>

wiresheet.py

@@ -180,10 +180,16 @@ def draw_knob(x, y):
     )
 
 
+LINK_SELECT_COLOR = "#e00000"
+LINK_SELECT_WIDTH = 2
+
+
 def draw_links():
     """Paints wires between slots on this sheet and knobs for links leaving it."""
     canvas = app_state.canvas
     canvas.delete("link")
+    app_state.canvas_link_map.clear()
+    app_state.selected_link_tag = None
     if app_state.xml_root_element is None:
         return
 
@@ -200,6 +206,7 @@ def draw_links():
 
     boxes = [b for b in (get_box_bounds(t) for t in path_to_tag.values()) if b]
 
+    link_index = 0
     for link in links_elem.findall("link"):
         from_ref = link.get("from", "")
         to_ref = link.get("to", "")
@@ -218,19 +225,21 @@ def draw_links():
             if src_box is None or dst_box is None:
                 continue
 
+            wire_tag = f"link_wire_{link_index}"
+            link_index += 1
+            app_state.canvas_link_map[wire_tag] = link
+
             points = route_link_path(src_box, src_y, dst_box, dst_y, boxes)
             flat = [coord for point in points for coord in point]
             canvas.create_line(
                 *flat, fill=LINK_COLOR, width=1,
-                arrow=tk.LAST, arrowshape=(8, 9, 3), tags=("link",)
+                arrow=tk.LAST, arrowshape=(8, 9, 3), tags=("link", wire_tag)
             )
         elif src_tag:
-            # Source is here, target is on another sheet.
             src_box, src_y = resolve_slot_anchor(src_tag, from_slot)
             if src_box is not None:
                 draw_knob(src_box[2] + KNOB_RADIUS, src_y)
         elif dst_tag:
-            # Target is here, source is on another sheet.
             dst_box, dst_y = resolve_slot_anchor(dst_tag, to_slot)
             if dst_box is not None:
                 draw_knob(dst_box[0] - KNOB_RADIUS, dst_y)
@@ -240,6 +249,27 @@ def draw_links():
         canvas.tag_lower("link", "component")
 
 
+def select_link(wire_tag):
+    """Highlights a link wire in red."""
+    canvas = app_state.canvas
+    deselect_link()
+    if wire_tag and wire_tag in app_state.canvas_link_map:
+        canvas.itemconfig(wire_tag, fill=LINK_SELECT_COLOR, width=LINK_SELECT_WIDTH)
+        canvas.tag_raise(wire_tag)
+        app_state.selected_link_tag = wire_tag
+
+
+def deselect_link():
+    """Resets the selected link wire back to its default colour."""
+    canvas = app_state.canvas
+    if app_state.selected_link_tag:
+        try:
+            canvas.itemconfig(app_state.selected_link_tag, fill=LINK_COLOR, width=1)
+        except tk.TclError:
+            pass
+        app_state.selected_link_tag = None
+
+
 # --- Component painting ---
 
 def render_components(components_to_render):