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-architecture-overview.md

@@ -171,12 +171,12 @@ JoinIR ラインで守るべきルールを先に書いておくよ：
     - 変数再接続はヘッダ PHI の dst を使って `builder.variable_map` を更新（Reconnector）。
     - expr 用の PHI には一切触れない（carrier 専用ライン）。
 
-- **JoinInlineBoundaryBuilder（Phase 200-2 追加）**
+- **JoinInlineBoundaryBuilder（Phase 200-2 / Phase 201 完了）**
   - ファイル: `src/mir/join_ir/lowering/inline_boundary_builder.rs`
   - 責務:
     - JoinInlineBoundary の構築を Builder パターンで統一化。
     - フィールド直書きの散乱を防ぎ、inputs/outputs/condition_bindings/exit_bindings/loop_var_name/expr_result の設定を fluent API で実施。
-    - Pattern2 で試験導入済み（Pattern3/4 は将来適用予定）。
+    - **Phase 201 で Pattern1/2/3/4 全てに適用完了**（境界情報組み立てを 1 箇所に集約）。
 
 - **JoinIRVerifier（Phase 200-3 追加）**
   - ファイル: `src/mir/builder/control_flow/joinir/merge/mod.rs`（debug_assertions 専用関数）