feat(repl): Phase 288.1 session persistence + auto-display

実装内容:
- AST Rewriter (~430行): 未宣言変数を __repl.get/set に変換
- ExternCall Bridge: VM で __repl.get/set ハンドラー実装
- Rc<RefCell<>> セッション共有: VM と REPL runner 間で永続化
- 式自動表示: pure expression の結果を自動出力
- _ 変数: 最後の表示値を保存（Void は除外）
- .reset 実装: セッション変数の完全クリア
- Fail-Fast: 未定義変数読み取りで明示的エラー + ヒント

変更ファイル (8ファイル, +592行):
- src/runner/repl/ast_rewriter.rs (NEW, +430行)
- src/runner/repl/repl_runner.rs (+84/-35行)
- src/backend/mir_interpreter/handlers/externals.rs (+54行)
- src/mir/builder/calls/build.rs (+41行)
- src/backend/mir_interpreter/mod.rs (+12行)
- src/runner/repl/repl_session.rs (+11/-9行)
- src/runner/repl/mod.rs (+2行)
- src/runner/mod.rs (+2/-1行)

REPL専用設計（src/mir/builder/calls/build.rs の特別扱い理由）:
- __repl.get/set は REPL mode 専用の橋渡し機能
- try_build_repl_method_call() で早期検出・ExternCall 変換
- file mode では決して使用されない（VM で "outside REPL mode" エラー）
- 将来的にも file mode への影響ゼロを保証

検証済み:
- 変数永続化: x = 42; print(x) → 42 ✅
- 式自動表示: 1 + 1 → 2 ✅
- _ 変数: 10 * 2 → 20; _ → 20 ✅
- Fail-Fast: 未定義エラー + ヒント ✅
- 回帰テスト: 154/154 PASS ✅

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/backend/mir_interpreter/handlers/externals.rs

@@ -264,6 +264,60 @@ impl MirInterpreter {
                 }
                 return Err(self.err_invalid("hostbridge.extern_invoke unsupported [externals]"));
             }
+
+            // Phase 288.1: REPL session variable bridge
+            ("__repl", "get") => {
+                // args: [name: String]
+                if args.len() != 1 {
+                    return Err(self.err_invalid(format!(
+                        "__repl.get expects 1 argument, got {}",
+                        args.len()
+                    )));
+                }
+
+                let name = self.reg_load(args[0])?.to_string();
+
+                // REPL session から取得
+                if let Some(session) = &self.repl_session {
+                    // Clone the value before the borrow ends to avoid borrowing conflicts
+                    let value_opt = session.borrow().get(&name).cloned();
+                    match value_opt {
+                        Some(value) => {
+                            self.write_result(dst, value);
+                            Ok(())
+                        }
+                        None => Err(self.err_invalid(format!(
+                            "Undefined variable: '{}'\nHint: Variable not defined. Assign a value first.",
+                            name
+                        ))),
+                    }
+                } else {
+                    Err(self.err_invalid("__repl.get called outside REPL mode".to_string()))
+                }
+            }
+
+            ("__repl", "set") => {
+                // args: [name: String, value: Any]
+                if args.len() != 2 {
+                    return Err(self.err_invalid(format!(
+                        "__repl.set expects 2 arguments, got {}",
+                        args.len()
+                    )));
+                }
+
+                let name = self.reg_load(args[0])?.to_string();
+                let value = self.reg_load(args[1])?.clone();
+
+                // REPL session に保存
+                if let Some(session) = &self.repl_session {
+                    session.borrow_mut().set(name, value);
+                    self.write_void(dst);
+                    Ok(())
+                } else {
+                    Err(self.err_invalid("__repl.set called outside REPL mode".to_string()))
+                }
+            }
+
             _ => Err(self.err_invalid(format!("ExternCall {}.{} not supported", iface, method))),
         }
     }

src/backend/mir_interpreter/mod.rs

@@ -6,7 +6,9 @@
  * Print/Debug (best-effort), Barrier/Safepoint (no-op).
  */
 
+use std::cell::RefCell;
 use std::collections::{BTreeMap, HashMap};
+use std::rc::Rc;
 
 use crate::box_trait::NyashBox;
 
@@ -46,6 +48,9 @@ pub struct MirInterpreter {
     /// Call stack depth (exec_function_inner nesting). Used as a safety valve
     /// to prevent Rust stack overflow on accidental infinite recursion in MIR.
     pub(super) call_depth: usize,
+    /// Phase 288.1: REPL session reference (REPL mode only)
+    /// Enables variable persistence across REPL lines via __repl.get/set bridge
+    pub(super) repl_session: Option<Rc<RefCell<crate::runner::repl::ReplSessionBox>>>,
 }
 
 impl MirInterpreter {
@@ -64,6 +69,7 @@ impl MirInterpreter {
             branch_count: 0,
             compare_count: 0,
             call_depth: 0,
+            repl_session: None,
         }
     }
 
@@ -95,6 +101,12 @@ impl MirInterpreter {
             .detect_from_mir_functions(self.functions.keys());
     }
 
+    /// Phase 288.1: Set REPL session for variable persistence
+    /// Enables __repl.get/set ExternCall handlers to access session state
+    pub fn set_repl_session(&mut self, session: Rc<RefCell<crate::runner::repl::ReplSessionBox>>) {
+        self.repl_session = Some(session);
+    }
+
     /// Execute a BoxCall with VM's complete semantics (Phase 27-shortterm S-5.2-improved)
     ///
     /// This wrapper allows external modules (e.g., JoinIR Runner) to invoke BoxCall