test: 各 Pass 模块补模块级单测，测试随实现拆分

crates/oxide_cfg/src/finalize.rs

@@ -19,3 +19,38 @@ pub(super) fn finalize(mut blocks: Vec<BasicBlock>, exit_id: usize) -> Cfg {
 
     Cfg { blocks, entry: 0, exit: exit_id }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::{BasicBlock, EdgeKind};
+
+    /// exit 哨兵恒在 blocks 末尾（空块 0..0），preds 反推正确。
+    #[test]
+    fn exit_sentinel_appended_with_preds_reversed() {
+        let blocks_in = vec![
+            BasicBlock { inst_range: 0..1, preds: Vec::new(), succs: vec![(1, EdgeKind::Jump)] },
+            BasicBlock { inst_range: 1..2, preds: Vec::new(), succs: vec![(2, EdgeKind::Fallthrough)] },
+        ];
+        let cfg = finalize(blocks_in, 2);
+        assert_eq!(cfg.blocks.len(), 3, "2 实块 + exit 哨兵");
+        assert_eq!(cfg.entry, 0);
+        assert_eq!(cfg.exit, 2);
+        assert_eq!(cfg.blocks[2].inst_range, 0..0, "exit 是空块");
+        assert_eq!(cfg.blocks[1].preds, vec![0], "块 1 preds 反推自块 0 的 Jump 边");
+        assert_eq!(cfg.blocks[2].preds, vec![1], "exit preds 反推自块 1 的 Fallthrough 边");
+    }
+
+    /// succs/preds 对称：total 边数一致（Pitfall 4 防御性约束）。
+    #[test]
+    fn preds_and_succs_symmetric() {
+        let blocks_in = vec![
+            BasicBlock { inst_range: 0..1, preds: Vec::new(), succs: vec![(1, EdgeKind::Jump)] },
+            BasicBlock { inst_range: 1..2, preds: Vec::new(), succs: vec![(2, EdgeKind::Fallthrough)] },
+        ];
+        let cfg = finalize(blocks_in, 2);
+        let succs_total: usize = cfg.blocks.iter().map(|b| b.succs.len()).sum();
+        let preds_total: usize = cfg.blocks.iter().map(|b| b.preds.len()).sum();
+        assert_eq!(succs_total, preds_total, "succs/preds 对称");
+    }
+}