Fix topological sort head node initialization

CRITICAL BUG: Our implementation only added the FIRST head node to the ready
set and kept the rest as seeds. ODGI explicitly adds ALL head nodes at once:

From ODGI topological_sort.cpp lines 121-127:
"Dump all the heads into the oriented set, rather than having them as seeds.
We will only go for cycle-breaking seeds when we run out of heads."

This fundamental difference affected the traversal order and could cause
incorrect results in graphs with multiple connected components.

Fixed by adding ALL head (or tail) nodes to the ready set during
initialization, matching ODGI's behavior exactly.

Author: ekg

src/bidirected_ops.rs

@@ -1438,39 +1438,27 @@ impl BidirectedGraph {
         let mut masked_edges = HashSet::new();
 
         // Initialize with heads or tails if requested
-        // Important: Only add ONE head at a time to maintain path ordering
+        // ODGI: "Dump all the heads into the oriented set, rather than having them as seeds."
+        // This ensures we process all heads before any cycle-breaking seeds
         if use_heads {
-            // Get heads in path-order (already sorted by find_head_nodes)
             let heads = self.find_head_nodes();
-            if !heads.is_empty() {
-                // Start with the first head (earliest in paths)
-                let first_head = heads[0];
-                if verbose {
-                    eprintln!("[exact_odgi] Starting with head: node {}", first_head.node_id());
-                }
-                s.insert(first_head);
-                unvisited.remove(&first_head);
-                unvisited.remove(&first_head.flip());
-
-                // Keep remaining heads as seeds for later
-                for handle in heads.into_iter().skip(1) {
-                    seeds.push(handle);
-                }
+            if verbose && !heads.is_empty() {
+                eprintln!("[exact_odgi] Adding {} head nodes to ready set", heads.len());
+            }
+            for head in heads {
+                s.insert(head);
+                unvisited.remove(&head);
+                unvisited.remove(&head.flip());
             }
         } else if use_tails {
             let tails = self.find_tail_nodes();
-            if !tails.is_empty() {
-                let first_tail = tails[0];
-                if verbose {
-                    eprintln!("[exact_odgi] Starting with tail: node {}", first_tail.node_id());
-                }
-                s.insert(first_tail);
-                unvisited.remove(&first_tail);
-                unvisited.remove(&first_tail.flip());
-
-                for handle in tails.into_iter().skip(1) {
-                    seeds.push(handle);
-                }
+            if verbose && !tails.is_empty() {
+                eprintln!("[exact_odgi] Adding {} tail nodes to ready set", tails.len());
+            }
+            for tail in tails {
+                s.insert(tail);
+                unvisited.remove(&tail);
+                unvisited.remove(&tail.flip());
             }
         }