docs(joinir): phase80 plan finalize + fix p3 cond_env mut (dev-only)

Phase 80 後処理:
- phase80-bindingid-p3p4-plan.md: Status を Completed に更新、Task 80-B/C/D チェックマーク追加
- pattern3_with_if_phi.rs: cond_env を mut に修正 + #[allow(unused_mut)] 追加（将来の登録処理に備える）

Phase 80 完全完了（commit 84129a7e）: Pattern2/3/4 全て BindingId 配線完了
Next: Phase 81 - Pattern2 ExitLine contract stabilization

src/mir/builder/control_flow/joinir/patterns/pattern3_with_if_phi.rs

@@ -119,7 +119,8 @@ impl MirBuilder {
         let loop_var_name = ctx.loop_var_name.clone();
         let loop_var_id = ctx.loop_var_id;
 
-        let (cond_env, condition_bindings, _loop_var_join_id) =
+        #[allow(unused_mut)]
+        let (mut cond_env, condition_bindings, _loop_var_join_id) =
             ConditionEnvBuilder::build_for_break_condition_v2(
                 condition,
                 &loop_var_name,