feat(phase40-3): dev flag実装 + テストインフラ準備

Phase 40-3成果：
- dev flag追加: use_joinir_for_array_filter() (env.rs)
- テストドキュメント更新 (phase40_array_ext_filter_test.rs)

フルパイプライン統合はPhase 40-3.5+に延期

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

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

src/config/env.rs

@@ -265,6 +265,16 @@ pub fn joinir_debug_level() -> u8 {
     0
 }
 
+/// Phase 40-3: array_ext.filter JoinIR経路切り替えフラグ
+///
+/// - OFF (default): 旧AST→MIR + collect_assigned_vars経路
+/// - ON: JoinIR Frontend + JoinFuncMeta + convert_with_meta経路
+///
+/// Set HAKO_JOINIR_ARRAY_FILTER=1 to enable JoinIR route for array_ext.filter.
+pub fn use_joinir_for_array_filter() -> bool {
+    env_bool("HAKO_JOINIR_ARRAY_FILTER")
+}
+
 // VM legacy by-name call fallback was removed (Phase 2 complete).
 
 // ---- Phase 11.8 MIR cleanup toggles ----