feat(joinir): Phase 49-4 multi-target routing with graceful fallback

- Add ArrayExtBox.filter/2 as second JoinIR mainline target
- Fix function name arity: print_tokens is /0 (no implicit me in arity)
- Construct proper JSON v0 format with defs array for JoinIR Frontend
- Add catch_unwind for graceful fallback on unsupported patterns
- Add 3 array_filter tests (smoke, fallback, A/B comparison)
- All 6 Phase 49 tests passing

Dev flags:
- HAKO_JOINIR_PRINT_TOKENS_MAIN=1: JsonTokenizer.print_tokens/0
- HAKO_JOINIR_ARRAY_FILTER_MAIN=1: ArrayExtBox.filter/2

Note: Currently all loops fall back to legacy LoopBuilder due to
JoinIR Frontend expecting hardcoded variable names (i, acc, n).
Full JoinIR integration pending variable scope support in Phase 50+.

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

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

src/tests/joinir/mainline_phase49.rs

@@ -1,4 +1,4 @@
-// Phase 49-3.2: JoinIR Frontend Mainline Integration Test
+// Phase 49: JoinIR Frontend Mainline Integration Test
 //
 // このテストは cf_loop の JoinIR Frontend mainline route が
 // 正常に動作することを確認する。
@@ -7,8 +7,13 @@
 // - merge_joinir_mir_blocks() によるブロックマージ
 // - A/B 比較テスト（Route A vs Route B）
 //
+// Phase 49-4 実装済み:
+// - ArrayExtBox.filter/2 対応
+// - HAKO_JOINIR_ARRAY_FILTER_MAIN=1 dev フラグ追加
+//
 // テスト方法:
 // HAKO_JOINIR_PRINT_TOKENS_MAIN=1 cargo test --release joinir_mainline_phase49
+// HAKO_JOINIR_ARRAY_FILTER_MAIN=1 cargo test --release phase49_joinir_array_filter
 
 use crate::ast::ASTNode;
 use crate::mir::MirCompiler;
@@ -222,3 +227,201 @@ static box Main {
     std::env::remove_var("HAKO_PARSER_STAGE3");
     std::env::remove_var("NYASH_DISABLE_PLUGINS");
 }
+
+// =============================================================================
+// Phase 49-4: ArrayExtBox.filter/2 Tests
+// =============================================================================
+
+/// Phase 49-4: JoinIR Frontend mainline パイプラインが
+/// ArrayExtBox.filter 関数のコンパイル時にクラッシュしないことを確認
+#[test]
+fn phase49_joinir_array_filter_smoke() {
+    // Phase 49-4 mainline route は dev フラグで制御
+    std::env::set_var("HAKO_JOINIR_ARRAY_FILTER_MAIN", "1");
+    std::env::set_var("NYASH_JOINIR_MAINLINE_DEBUG", "1");
+    std::env::set_var("NYASH_PARSER_STAGE3", "1");
+    std::env::set_var("HAKO_PARSER_STAGE3", "1");
+    std::env::set_var("NYASH_DISABLE_PLUGINS", "1");
+
+    // ArrayExtBox.filter 簡易実装（if-in-loop パターン）
+    // fn パラメータは pred に変更（予約語競合回避）
+    let src = r#"
+static box ArrayExtBox {
+    filter(arr, pred) {
+        local out = new ArrayBox()
+        local i = 0
+        local n = arr.size()
+        loop(i < n) {
+            local v = arr.get(i)
+            if pred(v) {
+                out.push(v)
+            }
+            i = i + 1
+        }
+        return out
+    }
+}
+
+static box Main {
+    main() {
+        return 0
+    }
+}
+"#;
+
+    let ast: ASTNode = NyashParser::parse_from_string(src)
+        .expect("phase49-4: parse failed");
+
+    let mut mc = MirCompiler::with_options(false);
+    let result = mc.compile(ast);
+
+    // パイプラインがクラッシュしないことを確認
+    assert!(
+        result.is_ok(),
+        "phase49-4 array_filter mainline compile should not crash: {:?}",
+        result.err()
+    );
+
+    // クリーンアップ
+    std::env::remove_var("HAKO_JOINIR_ARRAY_FILTER_MAIN");
+    std::env::remove_var("NYASH_JOINIR_MAINLINE_DEBUG");
+    std::env::remove_var("NYASH_PARSER_STAGE3");
+    std::env::remove_var("HAKO_PARSER_STAGE3");
+    std::env::remove_var("NYASH_DISABLE_PLUGINS");
+}
+
+/// Phase 49-4: dev フラグ OFF 時は従来経路を使用することを確認
+#[test]
+fn phase49_joinir_array_filter_fallback() {
+    // dev フラグ OFF
+    std::env::remove_var("HAKO_JOINIR_ARRAY_FILTER_MAIN");
+    std::env::set_var("NYASH_PARSER_STAGE3", "1");
+    std::env::set_var("HAKO_PARSER_STAGE3", "1");
+    std::env::set_var("NYASH_DISABLE_PLUGINS", "1");
+
+    let src = r#"
+static box ArrayExtBox {
+    filter(arr, pred) {
+        local out = new ArrayBox()
+        local i = 0
+        local n = arr.size()
+        loop(i < n) {
+            local v = arr.get(i)
+            if pred(v) {
+                out.push(v)
+            }
+            i = i + 1
+        }
+        return out
+    }
+}
+
+static box Main {
+    main() {
+        return 0
+    }
+}
+"#;
+
+    let ast: ASTNode = NyashParser::parse_from_string(src)
+        .expect("phase49-4 fallback: parse failed");
+
+    let mut mc = MirCompiler::with_options(false);
+    let result = mc.compile(ast);
+
+    assert!(
+        result.is_ok(),
+        "phase49-4 fallback compile should succeed: {:?}",
+        result.err()
+    );
+
+    // クリーンアップ
+    std::env::remove_var("NYASH_PARSER_STAGE3");
+    std::env::remove_var("HAKO_PARSER_STAGE3");
+    std::env::remove_var("NYASH_DISABLE_PLUGINS");
+}
+
+/// Phase 49-4: A/B 比較テスト - Route A (legacy) vs Route B (JoinIR)
+/// ArrayExtBox.filter版
+#[test]
+fn phase49_joinir_array_filter_ab_comparison() {
+    let src = r#"
+static box ArrayExtBox {
+    filter(arr, pred) {
+        local out = new ArrayBox()
+        local i = 0
+        local n = arr.size()
+        loop(i < n) {
+            local v = arr.get(i)
+            if pred(v) {
+                out.push(v)
+            }
+            i = i + 1
+        }
+        return out
+    }
+}
+
+static box Main {
+    main() {
+        return 0
+    }
+}
+"#;
+
+    // Route A: Legacy path (flag OFF)
+    std::env::remove_var("HAKO_JOINIR_ARRAY_FILTER_MAIN");
+    std::env::set_var("NYASH_PARSER_STAGE3", "1");
+    std::env::set_var("HAKO_PARSER_STAGE3", "1");
+    std::env::set_var("NYASH_DISABLE_PLUGINS", "1");
+
+    let ast_a: ASTNode = NyashParser::parse_from_string(src)
+        .expect("phase49-4 A/B: parse failed (Route A)");
+    let mut mc_a = MirCompiler::with_options(false);
+    let result_a = mc_a.compile(ast_a);
+    assert!(
+        result_a.is_ok(),
+        "Route A compile should succeed: {:?}",
+        result_a.err()
+    );
+    let module_a = result_a.unwrap().module;
+    let blocks_a: usize = module_a
+        .functions
+        .values()
+        .map(|f| f.blocks.len())
+        .sum();
+
+    // Route B: JoinIR Frontend path (flag ON)
+    std::env::set_var("NYASH_PARSER_STAGE3", "1");
+    std::env::set_var("HAKO_PARSER_STAGE3", "1");
+    std::env::set_var("NYASH_DISABLE_PLUGINS", "1");
+    std::env::set_var("HAKO_JOINIR_ARRAY_FILTER_MAIN", "1");
+
+    let ast_b: ASTNode = NyashParser::parse_from_string(src)
+        .expect("phase49-4 A/B: parse failed (Route B)");
+    let mut mc_b = MirCompiler::with_options(false);
+    let result_b = mc_b.compile(ast_b);
+    assert!(
+        result_b.is_ok(),
+        "Route B compile should succeed: {:?}",
+        result_b.err()
+    );
+    let module_b = result_b.unwrap().module;
+    let blocks_b: usize = module_b
+        .functions
+        .values()
+        .map(|f| f.blocks.len())
+        .sum();
+
+    // Log block counts for debugging
+    eprintln!(
+        "[phase49-4 A/B filter] Route A: {} total blocks, Route B: {} total blocks",
+        blocks_a, blocks_b
+    );
+
+    // クリーンアップ
+    std::env::remove_var("HAKO_JOINIR_ARRAY_FILTER_MAIN");
+    std::env::remove_var("NYASH_PARSER_STAGE3");
+    std::env::remove_var("HAKO_PARSER_STAGE3");
+    std::env::remove_var("NYASH_DISABLE_PLUGINS");
+}