Cap the depth of a right-leaning tree too

Mutation testing found it: changing the right child's `depth + 1` to
`depth + 0` in the depth-first recursion survived the entire suite. The
depth cap was only ever tested against a chain that leans left, so
nothing said whether the right-hand descent counted at all -- a tree
that goes right for a million nodes would have recursed until the
process died, which is precisely what AKSL_TREE_MAX_DEPTH exists to
prevent.

The right-leaning chain is now tested in all three depth-first orders
and breadth-first, which carries its depth on the queue entry instead.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

tests/test_tree.c

@@ -413,6 +413,40 @@ static int test_tree_deeper_than_the_cap_is_out_of_bounds(void)
     AKSL_CHECK_OK(aksl_tree_iterate(&chain[0], &record_visit, NULL, NULL,
 				    AKSL_TREE_SEARCH_DFS_PREORDER, &log));
     AKSL_CHECK(log.count == AKSL_TREE_MAX_DEPTH);
+
+    /*
+     * And again leaning right. The recursion counts depth separately for each
+     * child, so a chain that only ever goes left says nothing about whether the
+     * right-hand descent counts at all -- mutation testing found exactly that:
+     * changing the right child's `depth + 1` to `depth + 0` survived the whole
+     * suite, because nothing here had ever recursed right more than three deep.
+     */
+    for ( i = 0; i < AKSL_TREE_MAX_DEPTH + 2; i++ ) {
+	AKSL_CHECK_OK(aksl_tree_node_init(&chain[i], NULL));
+    }
+    for ( i = 0; i < AKSL_TREE_MAX_DEPTH + 1; i++ ) {
+	chain[i].right = &chain[i + 1];
+    }
+    visitlog_init(&log);
+    AKSL_CHECK_STATUS(aksl_tree_iterate(&chain[0], &record_visit, NULL, NULL,
+					AKSL_TREE_SEARCH_DFS_PREORDER, &log),
+		      AKERR_OUTOFBOUNDS);
+
+    /* Both orders that descend right, so in-order and post-order count too. */
+    visitlog_init(&log);
+    AKSL_CHECK_STATUS(aksl_tree_iterate(&chain[0], &record_visit, NULL, NULL,
+					AKSL_TREE_SEARCH_DFS_INORDER, &log),
+		      AKERR_OUTOFBOUNDS);
+    visitlog_init(&log);
+    AKSL_CHECK_STATUS(aksl_tree_iterate(&chain[0], &record_visit, NULL, NULL,
+					AKSL_TREE_SEARCH_DFS_POSTORDER, &log),
+		      AKERR_OUTOFBOUNDS);
+
+    /* And breadth-first, whose depth is carried on the queue entry instead. */
+    visitlog_init(&log);
+    AKSL_CHECK_STATUS(aksl_tree_iterate(&chain[0], &record_visit, NULL, NULL,
+					AKSL_TREE_SEARCH_BFS, &log),
+		      AKERR_OUTOFBOUNDS);
     return 0;
 }