docs(repl): record Phase 288.1 contract and results

更新内容:
- docs/reference/language/repl.md
  - Phase 288.1 完了ステータスに更新
  - Implementation Architecture セクション追加（AST rewrite 方式説明）
  - AST Rewriter の動作フロー追加（Variable/Assignment 変換ロジック）
  - ExternCall Bridge の仕組み追加（__repl.get/set → VMValue）
  - Expression Detection ロジック追加（wrapper AST 判定）
  - 動作例を完全に更新（全機能が動作済み）

- docs/development/current/main/phases/phase-288/README.md
  - Phase 288.1 完了セクション追加
  - 変更ファイル一覧（8ファイル, +592行）記録
  - 確認コマンド 4種 記録（変数永続化/式表示/_変数/リセット）
  - 回帰テスト結果記録（154/154 PASS）

- docs/development/current/main/10-Now.md
  - "Phase 288.1 完了" に更新
  - 次の候補（REPL UX 改善 / JoinIR 設計作業）を追記

- CURRENT_TASK.md
  - 1段落サマリー更新（288.1 完了、次の方向性）

Phase 288.1 成果（SSOT記録）:
✅ 変数永続化（session → eval bridge）
✅ 式自動表示（pure expression auto-output）
✅ _ 変数（last displayed value）
✅ Fail-Fast 未定義エラー + ヒント
✅ セッションリセット（.reset）
✅ 154/154 smoke tests PASS（file mode 不変）

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

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

docs/reference/language/repl.md

@@ -1,6 +1,6 @@
 # REPL Mode (Read–Eval–Print Loop) — Specification (SSOT)
 
-**Status**: Phase 288 P0–P3 complete (2025-12-25, MVP) / Phase 288.1 next (session persistence + auto-display)
+**Status**: Phase 288.1 complete (2025-12-25, Session Persistence + Auto-Display)
 
 ## Philosophy: Two Execution Contexts, One Language
 
@@ -297,25 +297,53 @@ Recommended file-mode errors:
 
 ## 9) Phase 288 MVP Implementation Status
 
-### Completed (Phase 288 P0-P3) - 2025-12-25
+### Completed (Phase 288.1) - 2025-12-25
 
 ✅ **CLI Entry** (`--repl` / `-i` flags)
 ✅ **REPL Loop** (`.help`, `.exit`, `.reset` commands)
-✅ **ReplSessionBox** (VMValue-based session state)
+✅ **ReplSessionBox** (VMValue-based session state with `Rc<RefCell<>>`)
 ✅ **Implicit Local Binding** (`x = 1` compiles without error)
 ✅ **print() Output** (ExternCall output displays correctly)
-✅ **`_` Variable** (last value stored in session)
+✅ **Variable Persistence** (session variables accessible across lines via AST rewrite)
+✅ **Expression Auto-Display** (`1+1` → `2` automatic output)
+✅ **`_` Variable** (last displayed value accessible, Void not stored)
 ✅ **Session Reset** (`.reset` command clears state)
-✅ **Main Box Entry Point** (VM execution fixed)
+✅ **Fail-Fast Undefined Variables** (clear error messages with hints)
 
-### Deferred to Phase 288.1+
+### Implementation Architecture (Phase 288.1)
 
-⏳ **Variable Persistence** (session variables accessible across lines)
-⏳ **Expression Auto-Display** (`1+1` → `2` automatic output)
-⏳ **Complex Expression Detection** (distinguish expression vs statement)
-⏳ **REPL Variable Injection** (session → compilation context bridge)
+**AST Rewrite Approach**:
+```
+User Input: x = 42
+  ↓
+Parse → AST (static box Main { main() { x = 42 } })
+  ↓
+AST Rewriter (REPL-specific):
+  - Undeclared Variable: x → __repl.get("x")
+  - Undeclared Assignment: x = 42 → __repl.set("x", 42)
+  - Respects: local declarations, reserved names (me/true/false/null)
+  - Skips: nested scopes (function/method bodies), Main wrapper
+  ↓
+Rewritten AST → MIR Compiler
+  ↓
+ExternCall("__repl", "get"/"set", args)
+  ↓
+VM Execution → ReplSessionBox.get/set
+```
 
-### Current Behavior (Phase 288 P3 MVP)
+**Key Components**:
+- **`src/runner/repl/ast_rewriter.rs`** (~430 lines): AST transformation
+- **`src/mir/builder/calls/build.rs`**: `__repl.get/set` → ExternCall MIR lowering
+- **`src/backend/mir_interpreter/handlers/externals.rs`**: ExternCall handlers
+- **`Rc<RefCell<ReplSessionBox>>`**: Session sharing between REPL runner and VM
+
+**Expression Detection**:
+- Checks wrapper AST (`Main.main` body) for single expression node
+- Pure expressions: Literal, BinaryOp, MethodCall, New, etc.
+- Statements: Assignment, Local, Print, If, Loop, etc.
+- Void values not displayed or stored in `_`
+
+### Current Behavior (Phase 288.1)
 
 ```nyash
 >>> .help
@@ -325,33 +353,37 @@ Commands:
   .help - Show this help
 
 >>> x = 42
->>>                    # Silent (implicit local creation)
+>>>                    # Silent (implicit local creation via __repl.set)
 
->>> print("Hello REPL!")
-Hello REPL!            # print() output displays
-
->>> print(42 + 1)
-43                     # Arithmetic works
+>>> print(x)
+42                     # ✅ Variable persistence works!
 
 >>> 1 + 1
->>>                    # Silent (expression auto-display in Phase 288.1)
+2                      # ✅ Expression auto-display works!
+
+>>> _
+2                      # ✅ Last displayed value accessible
+
+>>> print("Hello")
+Hello                  # Statements don't auto-display
 
 >>> .reset
 Session reset
 
->>> print("After reset")
-After reset            # REPL continues after reset
+>>> print(x)
+Error: Runtime error: Invalid instruction: Undefined variable: 'x'
+Hint: Variable not defined. Assign a value first.
+>>>                    # ✅ Fail-Fast with clear error
 ```
 
-**Known Limitations** (Phase 288 MVP):
-- Variable persistence not yet implemented (`x = 1` then `print(x)` errors)
-- `_` variable stored but not accessible in user code yet
-- No expression auto-display (deferred to Phase 288.1)
-
-**Design Philosophy**: Follow 80/20 rule - deliver working REPL first, polish UX later.
+**Design Principles**:
+- **AST Rewrite**: REPL-specific transformation, parser unchanged
+- **ExternCall Bridge**: Clean VM/REPL separation
+- **Fail-Fast**: No silent void, clear error messages
+- **Box-First**: Complete isolation in `src/runner/repl/`
 
 ---
 
-**Document Version**: Phase 288 P0 (2025-12-25)
-**Implementation**: Phase 288 P1-P3 (CLI + Session + Basic UX)
-**Next Phase**: Phase 288.1 (Expression Auto-Display)
+**Document Version**: Phase 288.1 (2025-12-25)
+**Implementation**: Complete (CLI + Session + Persistence + Auto-Display)
+**Next**: Further UX improvements (multiline input, syntax highlighting, etc.)