feat(joinir): Phase 201 - JoinInlineBoundaryBuilder expansion to Pattern3/4

- Task 201-1: Established canonical Builder pattern documentation
  - Created docs/development/current/main/joinir-boundary-builder-pattern.md
  - Documented Builder usage patterns for all patterns (P1/P2/P3/P4)
  - Added reference comments in pattern lowerers

- Task 201-2: Refactored Pattern3 to use Builder (removed field mutations)
  - Replaced new_with_exit_bindings + field mutation with Builder chain
  - Pattern3: 2 carriers (i + sum), exit_bindings, loop_var_name
  - Proper LoopExitBinding struct usage

- Task 201-3: Refactored Pattern4 to use Builder (continue/Trim support)
  - Replaced new_with_exit_bindings + field mutation with Builder chain
  - Pattern4: Dynamic carrier count, proper boundary construction

- Task 201-4: Added unit tests for Pattern3/4 style boundaries
  - test_builder_pattern3_style: Two carriers, exit_bindings validation
  - test_builder_pattern4_style: Dynamic carrier count validation
  - Verified no field mutations remain (exit_binding.rs uses deprecated fields only)

- Task 201-5: Updated architecture docs and CURRENT_TASK
  - joinir-architecture-overview.md: Builder now applied to all patterns
  - CURRENT_TASK.md: Phase 201 completion entry

All patterns now use consistent boundary construction via Builder.
Tests: All patterns pass (挙動不変).

docs/development/current/main/joinir-boundary-builder-pattern.md

@@ -0,0 +1,95 @@
+# JoinInlineBoundaryBuilder 使用パターン（Phase 201）
+
+## Pattern2 の Canonical 形式
+
+Pattern2 で確立した Builder 使用パターンを他の Pattern に適用する際の基準。
+
+### Builder 構築順序
+
+```rust
+let boundary = JoinInlineBoundaryBuilder::new()
+    .with_inputs(join_inputs, host_inputs)
+    .with_condition_bindings(condition_bindings)
+    .with_exit_bindings(exit_bindings)
+    .with_loop_var_name(Some(loop_var_name.clone()))
+    .with_expr_result(fragment_meta.expr_result)
+    .build();
+```
+
+### 各メソッドの意味
+
+1. **with_inputs(join_inputs, host_inputs)**
+   - ループパラメータの橋渡し
+   - join_inputs: JoinIR 側の ValueId リスト
+   - host_inputs: MIR 側の ValueId リスト
+
+2. **with_condition_bindings(bindings)**
+   - 条件式専用変数の橋渡し
+   - ConditionBinding のリスト（変数名 + JoinIR ValueId のペア）
+
+3. **with_exit_bindings(bindings)**
+   - キャリア出口の橋渡し
+   - (変数名, JoinIR ValueId) のペア
+
+4. **with_loop_var_name(name)**
+   - ループ変数名（LoopHeader PHI 生成用）
+   - Some(変数名) または None
+
+5. **with_expr_result(expr)**
+   - ループが式として返す値
+   - Some(ValueId) または None
+
+### フィールド直書き禁止
+
+- ❌ `boundary.loop_var_name = ...;` 等の直接代入は禁止
+- ✅ Builder の fluent API のみ使用
+
+### Pattern別の使用パターン
+
+#### Pattern2 (with break)
+```rust
+let boundary = JoinInlineBoundaryBuilder::new()
+    .with_inputs(vec![ValueId(0)], vec![loop_var_id])
+    .with_condition_bindings(condition_bindings)
+    .with_exit_bindings(exit_bindings)
+    .with_loop_var_name(Some(loop_var_name.clone()))
+    .with_expr_result(fragment_meta.expr_result)
+    .build();
+```
+
+#### Pattern3 (with if-phi)
+```rust
+let boundary = JoinInlineBoundaryBuilder::new()
+    .with_inputs(
+        vec![ValueId(0), ValueId(1)],      // i, sum
+        vec![loop_var_id, sum_var_id],
+    )
+    .with_exit_bindings(vec![
+        ("sum".to_string(), ValueId(18))
+    ])
+    .with_loop_var_name(Some(loop_var_name.clone()))
+    .build();
+```
+
+#### Pattern4 (with continue)
+```rust
+let boundary = JoinInlineBoundaryBuilder::new()
+    .with_inputs(join_inputs, host_inputs)  // Dynamic carrier count
+    .with_exit_bindings(exit_bindings)
+    .with_loop_var_name(Some(loop_var_name.clone()))
+    .build();
+```
+
+## Phase 201 目標
+
+- ✅ Pattern2/3/4 全てで Builder 使用に統一
+- ✅ フィールド直書き（`boundary.field = value;`）完全排除
+- ✅ 境界情報の組み立てを 1 箇所（Builder）に集約
+- ✅ テスト全 PASS（挙動不変）
+
+## 実装ファイル
+
+- Builder 本体: `src/mir/join_ir/lowering/inline_boundary_builder.rs`
+- Pattern2: `src/mir/builder/control_flow/joinir/patterns/pattern2_with_break.rs`
+- Pattern3: `src/mir/builder/control_flow/joinir/patterns/pattern3_with_if_phi.rs`
+- Pattern4: `src/mir/builder/control_flow/joinir/patterns/pattern4_with_continue.rs`