Fix index-out-of-bounds panic in build_subgraph_layouts when subgraphs have no positioned nodes

SubgraphTree indexes into graph.subgraphs (0..N), but the local
subgraphs vec skips any subgraph whose nodes have no layout positions
yet, causing the two index spaces to diverge.

Fix: build a graph_to_local mapping during the filter pass, size
all_descendants by graph.subgraphs.len() so tree indices are always
valid, and translate through graph_to_local in the bounds-expansion
loop, skipping filtered-out subgraphs.

Regression test added: tests/fixtures/flowchart/subgraph_empty.mmd
reproduces a flowchart with a subgraph that has no positioned nodes.

(cherry picked from commit 1042029)

Author: codewarnab

src/layout/subgraphs.rs

@@ -1321,6 +1321,8 @@ pub(super) fn build_subgraph_layouts(
     config: &LayoutConfig,
 ) -> Vec<SubgraphLayout> {
     let mut subgraphs = Vec::new();
+    // Maps graph.subgraphs index -> local subgraphs index (None if skipped).
+    let mut graph_to_local: Vec<Option<usize>> = Vec::with_capacity(graph.subgraphs.len());
     for sub in &graph.subgraphs {
         let mut min_x = f32::MAX;
         let mut min_y = f32::MAX;
@@ -1337,6 +1339,7 @@ pub(super) fn build_subgraph_layouts(
         }
 
         if min_x == f32::MAX {
+            graph_to_local.push(None);
             continue;
         }
 
@@ -1360,6 +1363,7 @@ pub(super) fn build_subgraph_layouts(
         let width = base_width.max(min_label_width);
         let extra_width = width - base_width;
 
+        graph_to_local.push(Some(subgraphs.len()));
         subgraphs.push(SubgraphLayout {
             label: sub.label.clone(),
             label_block,
@@ -1375,8 +1379,9 @@ pub(super) fn build_subgraph_layouts(
 
     if subgraphs.len() > 1 {
         let tree = SubgraphTree::build(graph);
-        let mut all_descendants: Vec<Vec<usize>> = vec![Vec::new(); subgraphs.len()];
-        let mut order: Vec<usize> = Vec::with_capacity(subgraphs.len());
+        let n = graph.subgraphs.len();
+        let mut all_descendants: Vec<Vec<usize>> = vec![Vec::new(); n];
+        let mut order: Vec<usize> = Vec::with_capacity(n);
         let mut stack: Vec<(usize, bool)> =
             tree.top_level.iter().rev().map(|&i| (i, false)).collect();
         while let Some((idx, visited)) = stack.pop() {
@@ -1400,20 +1405,26 @@ pub(super) fn build_subgraph_layouts(
         }
 
         for &i in &order {
+            let Some(local_i) = graph_to_local[i] else {
+                continue;
+            };
             for &j in &all_descendants[i] {
                 if is_region_subgraph(&graph.subgraphs[j]) {
                     continue;
                 }
+                let Some(local_j) = graph_to_local[j] else {
+                    continue;
+                };
                 let pad = if graph.kind == crate::ir::DiagramKind::State {
                     (theme.font_size * 1.8).max(24.0)
                 } else {
                     12.0
                 };
                 let (child_x, child_y, child_w, child_h) = {
-                    let child = &subgraphs[j];
+                    let child = &subgraphs[local_j];
                     (child.x, child.y, child.width, child.height)
                 };
-                let parent = &mut subgraphs[i];
+                let parent = &mut subgraphs[local_i];
                 let min_x = parent.x.min(child_x - pad);
                 let min_y = parent.y.min(child_y - pad);
                 let max_x = (parent.x + parent.width).max(child_x + child_w + pad);

tests/fixtures/flowchart/subgraph_empty.mmd

@@ -0,0 +1,20 @@
+flowchart TD
+    subgraph ROW1["Input"]
+        direction LR
+        A["Node A"]
+        B["Node B"]
+    end
+    subgraph ROW2["Processing"]
+        direction LR
+        C["Node C"]
+        D["Node D"]
+    end
+    subgraph ROW3["Annotations"]
+        direction LR
+        NOTE1("This subgraph has only\nannotation nodes that\nhave no positioned peers")
+        NOTE2("Another annotation\nwithout a main node")
+        NOTE1 -.- NOTE2
+    end
+    A --> C
+    B --> D
+    NOTE1 -.- A

tests/layout_suite.rs

@@ -316,6 +316,7 @@ fn render_all_fixtures() {
         "flowchart/styles.mmd",
         "flowchart/subgraph.mmd",
         "flowchart/subgraph_direction.mmd",
+        "flowchart/subgraph_empty.mmd",
         "flowchart/cycles.mmd",
         "gantt/basic.mmd",
         "gitgraph/basic.mmd",