Fix page range bug: clamp end_index >= start_index

When consecutive sections start on the same page, end_index could
become less than start_index (e.g. pages 10-9), resulting in empty
context text. Clamp end_index to at least start_index so every
section has at least one page of content.

Author: mithun50

src/tree-builder.ts

@@ -53,6 +53,11 @@ function assignRanges(nodes: TreeNode[], boundaryEnd: number): void {
       node.end_index = boundaryEnd;
     }
 
+    // Clamp: end must be >= start (sections on the same page)
+    if (node.end_index < node.start_index) {
+      node.end_index = node.start_index;
+    }
+
     if (node.nodes.length > 0) {
       assignRanges(node.nodes, node.end_index);
     }

treedex/tree_builder.py

@@ -37,6 +37,10 @@ def _assign_ranges(nodes: list[dict], boundary_end: int):
         else:
             node["end_index"] = boundary_end
 
+        # Clamp: end must be >= start (sections on the same page)
+        if node["end_index"] < node["start_index"]:
+            node["end_index"] = node["start_index"]
+
         if node.get("nodes"):
             _assign_ranges(node["nodes"], node["end_index"])