fix: use right_id for prefix check in structure_cost computation

The transition cost is conn_cost(prev.right_id, next.left_id), so
the prefix check should use right_id (the outgoing POS) rather than
left_id (the incoming POS) of the previous segment.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: send

engine/crates/lex-core/src/converter/reranker.rs

@@ -123,7 +123,7 @@ pub fn rerank(
             for i in 1..p.segments.len() {
                 let mut tc = conn_cost(conn, p.segments[i - 1].right_id, p.segments[i].left_id);
                 if let Some(c) = conn {
-                    if c.is_prefix(p.segments[i - 1].left_id) {
+                    if c.is_prefix(p.segments[i - 1].right_id) {
                         tc = tc.max(prefix_floor);
                     }
                 }