feat(joinir): Phase 33-8 Stage-1 rollout infrastructure

Add environment variable controls and debug logging for JoinIR lowering rollout.

Changes:
- Add HAKO_JOINIR_STAGE1 env var for Stage-1 function rollout control
- Add HAKO_JOINIR_DEBUG (0-3) for granular debug logging
  - Level 0: Silent (default)
  - Level 1: Basic lowering info
  - Level 2: Pattern matching details
  - Level 3: Full variable/instruction dump
- Implement 3-tier whitelist system:
  - Tier 1: Test functions (always enabled)
  - Tier 2: Stage-1 rollout (env-controlled)
  - Tier 3: Explicit approvals (validated in Phase 33-4)
- Add A/B test automation script (tools/joinir_ab_test.sh)
- Update if_merge.rs and if_select.rs with debug_level support

Environment variables (with NYASH_* fallback for compatibility):
- HAKO_JOINIR_IF_SELECT: Enable JoinIR lowering
- HAKO_JOINIR_STAGE1: Enable Stage-1 function rollout
- HAKO_JOINIR_DEBUG: Debug log level (0-3)

A/B test verification: PASSED on joinir_if_merge_{simple,multiple}.hako

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

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

src/mir/join_ir/lowering/mod.rs

@@ -73,15 +73,29 @@ pub fn try_lower_if_to_joinir(
         return None;
     }
 
-    // 2. Phase 33-7: 関数名ガード拡張（テスト + Stage-1/Stage-B 候補）
-    let is_allowed = func.signature.name.starts_with("IfSelectTest.")
-        || func.signature.name.starts_with("IfMergeTest.") // Phase 33-7
-        || func.signature.name.starts_with("Stage1JsonScannerTestBox.") // Phase 33-5 test
-        || func.signature.name == "JsonShapeToMap._read_value_from_pair/1"
-        || func.signature.name == "Stage1JsonScannerBox.value_start_after_key_pos/2";
+    // Phase 33-8: デバッグログレベル取得（0-3）
+    let debug_level = crate::config::env::joinir_debug_level();
+    let _debug = debug || debug_level >= 1;
+
+    // 2. Phase 33-8: 関数名ガード拡張（テスト + Stage-1 rollout + 明示承認）
+    let is_allowed =
+        // Test functions (always enabled)
+        func.signature.name.starts_with("IfSelectTest.") ||
+        func.signature.name.starts_with("IfMergeTest.") ||
+        func.signature.name.starts_with("Stage1JsonScannerTestBox.") || // Phase 33-5 test
+
+        // Stage-1 rollout (env-controlled)
+        (crate::config::env::joinir_stage1_enabled() &&
+         func.signature.name.starts_with("Stage1")) ||
+
+        // Explicit approvals (Phase 33-4で検証済み, always on)
+        matches!(func.signature.name.as_str(),
+            "JsonShapeToMap._read_value_from_pair/1" |
+            "Stage1JsonScannerBox.value_start_after_key_pos/2"
+        );
 
     if !is_allowed {
-        if debug {
+        if debug_level >= 2 {
             eprintln!(
                 "[try_lower_if_to_joinir] skipping non-allowed function: {}",
                 func.signature.name
@@ -90,15 +104,19 @@ pub fn try_lower_if_to_joinir(
         return None;
     }
 
+    if debug_level >= 1 {
+        eprintln!("[try_lower_if_to_joinir] trying to lower {}", func.signature.name);
+    }
+
     // 3. Phase 33-7: IfMerge を優先的に試行（複数変数パターン）
     //    IfMerge が成功すればそれを返す、失敗したら Select を試行
-    let if_merge_lowerer = if_merge::IfMergeLowerer::new(debug);
+    let if_merge_lowerer = if_merge::IfMergeLowerer::new(debug_level);
 
     if if_merge_lowerer.can_lower_to_if_merge(func, block_id) {
         if let Some(result) = if_merge_lowerer.lower_if_to_if_merge(func, block_id) {
-            if debug {
+            if debug_level >= 1 {
                 eprintln!(
-                    "[try_lower_if_to_joinir] IfMerge lowering used for {}",
+                    "[try_lower_if_to_joinir] ✅ IfMerge lowering used for {}",
                     func.signature.name
                 );
             }
@@ -107,10 +125,10 @@ pub fn try_lower_if_to_joinir(
     }
 
     // 4. IfMerge が失敗したら Select を試行（単一変数パターン）
-    let if_select_lowerer = if_select::IfSelectLowerer::new(debug);
+    let if_select_lowerer = if_select::IfSelectLowerer::new(debug_level);
 
     if !if_select_lowerer.can_lower_to_select(func, block_id) {
-        if debug {
+        if debug_level >= 1 {
             eprintln!(
                 "[try_lower_if_to_joinir] pattern not matched for {}",
                 func.signature.name
@@ -121,9 +139,9 @@ pub fn try_lower_if_to_joinir(
 
     let result = if_select_lowerer.lower_if_to_select(func, block_id);
 
-    if result.is_some() && debug {
+    if result.is_some() && debug_level >= 1 {
         eprintln!(
-            "[try_lower_if_to_joinir] Select lowering used for {}",
+            "[try_lower_if_to_joinir] ✅ Select lowering used for {}",
             func.signature.name
         );
     }