refactor(cfg): 拆分 build_cfg 四阶段为独立模块纯函数链

crates/oxide_cfg/src/finalize.rs

@@ -0,0 +1,21 @@
+//! Pass 4：exit 哨兵收尾 + succs 反推 preds。
+
+use crate::{BasicBlock, Cfg};
+
+/// 追加 exit 哨兵块（`0..0`，置末尾），从各块 succs 反推 preds。
+pub(super) fn finalize(mut blocks: Vec<BasicBlock>, exit_id: usize) -> Cfg {
+    blocks.push(BasicBlock { inst_range: 0..0, preds: Vec::new(), succs: Vec::new() });
+    let mut succs_total = 0usize;
+    let mut preds_total = 0usize;
+    for i in 0..blocks.len() {
+        succs_total += blocks[i].succs.len();
+        let targets: Vec<crate::BBId> = blocks[i].succs.iter().map(|&(t, _)| t).collect();
+        for target in targets {
+            blocks[target].preds.push(i);
+            preds_total += 1;
+        }
+    }
+    debug_assert_eq!(succs_total, preds_total, "succs/preds 不对称（Pitfall 4）");
+
+    Cfg { blocks, entry: 0, exit: exit_id }
+}