Fix joining paths from different layers with Ctrl+J fails (#3534)

* fix: handle PointId remapping when joining paths from different layers

When using Ctrl+J to join two points from different layers, the operation
failed because PointIds change during merge due to collision resolution.

The fix uses index-based point tracking instead of PointIds:
1. Before merge: Record the index of each selected point in its layer
2. Calculate post-merge indices using the point offset from layer1
3. After merge: Retrieve the actual PointIds at those indices
4. Create the connecting segment using the resolved PointIds

This works because Vector::concat() preserves point ordering during merge,
so we can use exact index arithmetic to find points after remapping.

Fixes #3519

* fix: use position-based lookup when joining paths from different layers

* fix cmd + g group case

* fix check for grouped folder

* add in the single layer inside a group case

* use ApplyPointDelta instead of creating a dummy segment

* Update editor/src/messages/tool/common_functionality/shape_editor.rs

Co-authored-by: James Lindsay <78500760+0HyperCube@users.noreply.github.com>

* Update editor/src/messages/tool/common_functionality/shape_editor.rs

Co-authored-by: James Lindsay <78500760+0HyperCube@users.noreply.github.com>

* encapsulate point connection logic in defer_connect_points_by_position

* reduce tolerance to 1e-6

* remove wrong comments

---------

Co-authored-by: James Lindsay <78500760+0HyperCube@users.noreply.github.com>

editor/src/messages/tool/tool_messages/path_tool.rs

@@ -73,6 +73,11 @@ pub enum PathToolMessage {
 	},
 	Escape,
 	ClosePath,
+	ConnectPointsByPosition {
+		layer: LayerNodeIdentifier,
+		start_position: DVec2,
+		end_position: DVec2,
+	},
 	DoubleClick {
 		extend_selection: Key,
 		shrink_selection: Key,
@@ -2669,6 +2674,60 @@ impl Fsm for PathToolFsmState {
 
 				self
 			}
+			(_, PathToolMessage::ConnectPointsByPosition { layer, start_position, end_position }) => {
+				// Get the merged vector
+				let Some(vector) = document.network_interface.compute_modified_vector(layer) else {
+					return self;
+				};
+
+				// Find points by their positions (with small tolerance for floating point comparison)
+				const POSITION_TOLERANCE: f64 = 1e-6;
+
+				let positions = vector.point_domain.positions();
+				let point_ids = vector.point_domain.ids();
+
+				let mut start_point_id = None;
+				let mut end_point_id = None;
+
+				// Get the merged layer's transform to convert local positions to document space
+				let layer_transform = document.metadata().transform_to_document(layer);
+
+				for (i, &local_pos) in positions.iter().enumerate() {
+					// Transform the local position to document space for comparison
+					let doc_pos = layer_transform.transform_point2(local_pos);
+
+					let start_distance = (doc_pos - start_position).length();
+					let end_distance = (doc_pos - end_position).length();
+
+					if start_point_id.is_none() && start_distance < POSITION_TOLERANCE {
+						start_point_id = Some(point_ids[i]);
+					}
+					if end_point_id.is_none() && end_distance < POSITION_TOLERANCE {
+						end_point_id = Some(point_ids[i]);
+					}
+					if start_point_id.is_some() && end_point_id.is_some() {
+						break;
+					}
+				}
+
+				if let (Some(start_id), Some(end_id)) = (start_point_id, end_point_id) {
+					// Create segment directly
+					responses.add(DocumentMessage::StartTransaction);
+
+					let segment_id = SegmentId::generate();
+					let modification_type = VectorModificationType::InsertSegment {
+						id: segment_id,
+						points: [end_id, start_id],
+						handles: [None, None],
+					};
+
+					responses.add(GraphOperationMessage::Vector { layer, modification_type });
+					responses.add(DocumentMessage::EndTransaction);
+					responses.add(OverlaysMessage::Draw);
+				}
+
+				self
+			}
 			(_, PathToolMessage::StartSlidingPoint) => {
 				responses.add(DocumentMessage::StartTransaction);
 				if tool_data.start_sliding_point(shape_editor, document) {