feat(joinir): Phase 49-3.2 merge_joinir_mir_blocks full implementation

Implement actual block merging for JoinIR Frontend mainline integration:

- Block ID remapping: Allocate new IDs from block_gen for all JoinIR blocks
- Value ID remapping: Allocate new IDs from next_value_id() for all values
- Instruction cloning: Clone all instructions with remapped IDs
- Return→Jump conversion: Convert Return terminators to Jump to exit block
- Control flow wiring: Jump from current block to JoinIR entry

Helper functions added:
- collect_values_in_block(): Collect all ValueIds in a block
- collect_values_in_instruction(): Collect all ValueIds in an instruction
- remap_instruction(): Remap ValueIds and BlockIds in an instruction

A/B tests (3 total):
- phase49_joinir_mainline_pipeline_smoke
- phase49_joinir_mainline_fallback_without_flag
- phase49_joinir_mainline_ab_comparison (Route A vs Route B)

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

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

src/tests/joinir/mainline_phase49.rs

@@ -1,11 +1,11 @@
-// Phase 49-3: JoinIR Frontend Mainline Integration Test
+// Phase 49-3.2: 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。
 //
-// MVP 制限:
-// - merge_joinir_mir_blocks() はログ出力のみ
-// - 完全な A/B 比較は Phase 49-3.2（ブロックマージ実装）待ち
+// Phase 49-3.2 実装済み:
+// - 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）
 //
 // テスト方法:
 // HAKO_JOINIR_PRINT_TOKENS_MAIN=1 cargo test --release joinir_mainline_phase49
@@ -129,3 +129,96 @@ static box Main {
     std::env::remove_var("HAKO_PARSER_STAGE3");
     std::env::remove_var("NYASH_DISABLE_PLUGINS");
 }
+
+/// Phase 49-3.2: A/B 比較テスト - Route A (legacy) vs Route B (JoinIR)
+///
+/// このテストは同じソースコードを2つの経路でコンパイルし、
+/// 両方が正常に完了することを確認する。
+#[test]
+fn phase49_joinir_mainline_ab_comparison() {
+    let src = r#"
+box JsonTokenizer {
+    tokens: ArrayBox
+
+    birth() {
+        me.tokens = new ArrayBox()
+    }
+
+    print_tokens() {
+        local i = 0
+        loop(i < me.tokens.length()) {
+            i = i + 1
+        }
+    }
+}
+
+static box Main {
+    main() {
+        local t = new JsonTokenizer()
+        t.print_tokens()
+        return 0
+    }
+}
+"#;
+
+    // Route A: Legacy path (flag OFF)
+    std::env::remove_var("HAKO_JOINIR_PRINT_TOKENS_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 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)
+    // Re-set parser flags to ensure they're active
+    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_PRINT_TOKENS_MAIN", "1");
+
+    let ast_b: ASTNode = NyashParser::parse_from_string(src)
+        .expect("phase49 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 A/B] Route A: {} total blocks, Route B: {} total blocks",
+        blocks_a, blocks_b
+    );
+
+    // Both should complete successfully (main assertion is the compile succeeds)
+    // Block counts may differ due to JoinIR's different structure
+    // Future: Add execution comparison
+
+    // クリーンアップ
+    std::env::remove_var("HAKO_JOINIR_PRINT_TOKENS_MAIN");
+    std::env::remove_var("NYASH_PARSER_STAGE3");
+    std::env::remove_var("HAKO_PARSER_STAGE3");
+    std::env::remove_var("NYASH_DISABLE_PLUGINS");
+}