feat(joinir): Phase 34-7.5 helpers + Phase 34-8 Break/Continue implementation

Phase 34-7.5: Code organization improvements
- Added type conversion helpers (as_cont/as_func) in join_ir/mod.rs
- Enhanced docstrings for JoinCall/JoinJump with usage examples
- Improved error messages in join_ir_vm_bridge.rs

JoinIrFrontendTestRunner box implementation
- Created src/tests/helpers/joinir_frontend.rs (119 lines)
- Reduced test code by 83% (70 lines → 12 lines per test)
- 26% overall reduction in test file (284 → 209 lines)

Phase 34-8: Break/Continue pattern implementation
- Extended ast_lowerer.rs (+630 lines)
  - lower_loop_break_pattern(): Break as Jump (early return)
  - lower_loop_continue_pattern(): Continue as Select + Call
  - Added Bool literal support in extract_value()
- Created 2 fixtures: loop_frontend_{break,continue}.program.json
- Added 2 A/B tests (all 6 Phase 34 tests PASS)

Technical achievements:
- Break = Jump (early return pattern)
- Continue = Select + Call (NOT Jump) - critical discovery
- 3-function structure sufficient (no k_continue needed)
- SSA-style re-assignment with natural var_map updates

Test results:
✅ joinir_frontend_if_select_simple_ab_test
✅ joinir_frontend_if_select_local_ab_test
✅ joinir_frontend_json_shape_read_value_ab_test
✅ joinir_frontend_loop_simple_ab_test
✅ joinir_frontend_loop_break_ab_test
✅ joinir_frontend_loop_continue_ab_test

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

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

src/mir/join_ir/mod.rs

@@ -52,6 +52,23 @@ impl JoinFuncId {
     pub fn new(id: u32) -> Self {
         JoinFuncId(id)
     }
+
+    /// JoinFuncId を JoinContId に変換
+    ///
+    /// # Use Case
+    /// Jump 命令で関数を continuation として使う場合
+    /// ```rust
+    /// let func_id = JoinFuncId(42);
+    /// let cont_id = func_id.as_cont();
+    /// Jump { cont: cont_id, args: vec![], cond: None }
+    /// ```
+    ///
+    /// # Phase 34-7 Note
+    /// JoinFuncId と JoinContId は別の newtype だが、内部的には同じ u32 ID を共有する。
+    /// この変換は型レベルでの役割の明示（関数 vs 継続）を可能にする。
+    pub fn as_cont(self) -> JoinContId {
+        JoinContId(self.0)
+    }
 }
 
 /// 継続（join / ループ step / exit continuation）を識別するID
@@ -62,6 +79,21 @@ impl JoinContId {
     pub fn new(id: u32) -> Self {
         JoinContId(id)
     }
+
+    /// JoinContId を JoinFuncId に変換
+    ///
+    /// # Use Case
+    /// 継続 ID を関数 ID として参照する場合（JoinModule の functions map でルックアップ時など）
+    /// ```rust
+    /// let cont_id = JoinContId(42);
+    /// let func = join_module.functions.get(&cont_id.as_func())?;
+    /// ```
+    ///
+    /// # Phase 34-7 Note
+    /// JoinIR では継続も関数として実装されるため、この変換が必要になる。
+    pub fn as_func(self) -> JoinFuncId {
+        JoinFuncId(self.0)
+    }
 }
 
 /// 変数ID（Phase 26-H では MIR の ValueId を再利用）
@@ -179,7 +211,31 @@ pub struct MergePair {
 /// JoinIR 命令セット（最小版）
 #[derive(Debug, Clone)]
 pub enum JoinInst {
-    /// 通常の関数呼び出し: f(args..., k_next)
+    /// 通常の関数呼び出し（末尾再帰）: f(args..., k_next)
+    ///
+    /// # Semantics
+    /// - 他の JoinIR 関数を呼び出す（MIR の Call に変換）
+    /// - ループでは末尾再帰として使うのが典型的
+    ///
+    /// # MIR 変換
+    /// - `MirInstruction::Call { func, args, ... }` を生成
+    ///
+    /// # Constraints (Phase 31/34 時点)
+    /// - **k_next は常に None にすること！**
+    ///   JoinIR→MIR bridge が `k_next: Some(...)` 未対応
+    ///   → エラー: "Call with k_next is not yet supported"
+    /// - 典型的な使い方: `Call { func, args, k_next: None, dst: Some(...) }`
+    ///
+    /// # Loop Pattern での使い方 (Phase 34-7)
+    /// ```rust
+    /// // ✅ 正解: 末尾再帰
+    /// Call {
+    ///     func: loop_step_id,
+    ///     args: vec![i_next, acc_next, n],
+    ///     k_next: None,  // ⚠️ 必須: None にすること
+    ///     dst: Some(result),
+    /// }
+    /// ```
     Call {
         func: JoinFuncId,
         args: Vec<VarId>,
@@ -188,7 +244,46 @@ pub enum JoinInst {
         dst: Option<VarId>,
     },
 
-    /// 継続呼び出し（join / exit 継続など）
+    /// 継続呼び出し（早期 return / exit 継続）
+    ///
+    /// # Semantics
+    /// - **「早期 return」（条件付き関数脱出）として使う！**
+    /// - cond=Some(v): v が true なら cont に Jump、false なら次の命令へ
+    /// - cond=None: 無条件 Jump
+    ///
+    /// # MIR 変換
+    /// - cond=Some(v): `Branch(v, exit_block[Return], continue_block)` を生成
+    ///   - exit_block: cont 関数を Call して Return
+    ///   - continue_block: 次の JoinInst に続く
+    /// - cond=None: 無条件に cont を Call して Return
+    ///
+    /// # Loop Pattern での使い方 (Phase 34-7)
+    /// ```rust
+    /// // ✅ 正解: 条件付き早期 return
+    /// Jump {
+    ///     cont: k_exit_id.as_cont(),
+    ///     args: vec![acc],
+    ///     cond: Some(exit_cond),  // exit_cond が true なら k_exit へ
+    /// }
+    /// // ↑ exit_cond が false なら次の命令（body 処理）へ進む
+    ///
+    /// // ❌ 間違い: Call で条件分岐しようとする
+    /// Call {
+    ///     func: k_exit_id,
+    ///     cond: Some(exit_cond),  // こんなフィールドはない！
+    /// }
+    /// ```
+    ///
+    /// # 典型的なパターン
+    /// ```text
+    /// loop_step(i, acc, n):
+    ///   exit_cond = !(i < n)
+    ///   Jump(k_exit, [acc], cond=exit_cond)  // 🔑 早期 return
+    ///   // ↓ Jump で抜けなかった場合のみ実行
+    ///   acc_next = acc + 1
+    ///   i_next = i + 1
+    ///   Call(loop_step, [i_next, acc_next, n])  // 🔑 末尾再帰
+    /// ```
     Jump {
         cont: JoinContId,
         args: Vec<VarId>,