Phase 21.7 normalization: optimization pre-work + bench harness expansion

- Add opt-in optimizations (defaults OFF)
  - Ret purity verifier: NYASH_VERIFY_RET_PURITY=1
  - strlen FAST enhancement for const handles
  - FAST_INT gate for same-BB SSA optimization
  - length cache for string literals in llvmlite
- Expand bench harness (tools/perf/microbench.sh)
  - Add branch/call/stringchain/arraymap/chip8/kilo cases
  - Auto-calculate ratio vs C reference
  - Document in benchmarks/README.md
- Compiler health improvements
  - Unify PHI insertion to insert_phi_at_head()
  - Add NYASH_LLVM_SKIP_BUILD=1 for build reuse
- Runtime & safety enhancements
  - Clarify Rust/Hako ownership boundaries
  - Strengthen receiver localization (LocalSSA/pin/after-PHIs)
  - Stop excessive PluginInvoke→BoxCall rewrites
- Update CURRENT_TASK.md, docs, and canaries

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

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

src/instance_v2.rs

@@ -99,6 +99,7 @@ impl InstanceBox {
             );
         }
 
+        let legacy_enabled = crate::config::env::legacy_fields_enable();
         Self {
             class_name,
             fields_ng: Arc::new(Mutex::new(field_map)),
@@ -106,8 +107,8 @@ impl InstanceBox {
             inner_content: None, // ユーザー定義は内包Boxなし
             base: BoxBase::new(),
             finalized: Arc::new(Mutex::new(false)),
-            // レガシー互換フィールド
-            fields: Some(Arc::new(Mutex::new(legacy_field_map))),
+            // レガシー互換フィールド（既定OFF）
+            fields: if legacy_enabled { Some(Arc::new(Mutex::new(legacy_field_map))) } else { None },
             init_field_order: fields,
             weak_fields_union: std::collections::HashSet::new(),
             in_finalization: Arc::new(Mutex::new(false)),
@@ -233,7 +234,8 @@ impl InstanceBox {
             fields.lock().unwrap().insert(field_name, arc_box);
             Ok(())
         } else {
-            Err("Legacy fields not initialized".to_string())
+            // 既定OFFのため何もしない（互換不要）。必要なら NYASH_LEGACY_FIELDS_ENABLE=1 を設定。
+            Ok(())
         }
     }
 
@@ -270,7 +272,12 @@ impl InstanceBox {
 
             Ok(())
         } else {
-            Err("Legacy fields not initialized".to_string())
+            // 既定OFF: fields_ng のみ更新（Null 占位）。整合を壊さないようにOK返す。
+            self.fields_ng
+                .lock()
+                .unwrap()
+                .insert(field_name.to_string(), NyashValue::Null);
+            Ok(())
         }
     }