feat: using system完全実装完了！nyashstdビルトイン名前空間対応

✅ 実装内容:
- ビルトイン名前空間解決: nyashstd → builtin:nyashstd
- 自動コード生成: static box群（string/integer/bool/array/console）
- 環境変数デフォルト化: NYASH_ENABLE_USING, NYASH_RESOLVE_FIX_BRACES, NYASH_LLVM_USE_HARNESS

✅ 動作確認:
- パース→解決→読み込み→コード生成の全段階が正常動作
- 環境変数8個→6個に削減（25%改善）

✅ 主要変更:
- src/runner/pipeline.rs: builtin namespace特別処理追加
- src/runner/modes/common_util/resolve/strip.rs: builtin:プレフィックス処理
- src/config/env.rs: 3つの環境変数をデフォルトON化

🎯 次: Mini-VM開発でusing nyashstd活用可能

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

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

src/runner/modes/common_util/resolve/strip.rs

@@ -1,6 +1,56 @@
 use crate::runner::NyashRunner;
 use std::collections::HashSet;
 
+/// Generate content for built-in namespaces like builtin:nyashstd
+fn generate_builtin_namespace_content(namespace_key: &str) -> String {
+    match namespace_key {
+        "builtin:nyashstd" => {
+            // Generate Nyash code that provides nyashstd functionality
+            // This exposes the built-in stdlib boxes as regular Nyash static boxes
+            format!(r#"
+// Built-in nyashstd namespace (auto-generated)
+static box string {{
+    create(text) {{
+        return new StringBox(text)
+    }}
+    upper(str) {{
+        return new StringBox(str.upper())
+    }}
+}}
+
+static box integer {{
+    create(value) {{
+        return new IntegerBox(value)
+    }}
+}}
+
+static box bool {{
+    create(value) {{
+        return new BoolBox(value)
+    }}
+}}
+
+static box array {{
+    create() {{
+        return new ArrayBox()
+    }}
+}}
+
+static box console {{
+    log(message) {{
+        print(message)
+        return null
+    }}
+}}
+"#)
+        }
+        _ => {
+            // Unknown built-in namespace
+            format!("// Unknown built-in namespace: {}\n", namespace_key)
+        }
+    }
+}
+
 /// Strip `using` lines and register modules/aliases into the runtime registry.
 /// Returns cleaned source. No-op when `NYASH_ENABLE_USING` is not set.
 #[allow(dead_code)]
@@ -14,7 +64,7 @@ pub fn strip_using_and_register(
     }
     // Optional external combiner (default OFF): NYASH_USING_COMBINER=1
     if std::env::var("NYASH_USING_COMBINER").ok().as_deref() == Some("1") {
-        let fix_braces = std::env::var("NYASH_RESOLVE_FIX_BRACES").ok().as_deref() == Some("1");
+        let fix_braces = crate::config::env::resolve_fix_braces();
         let dedup_box = std::env::var("NYASH_RESOLVE_DEDUP_BOX").ok().as_deref() == Some("1");
         let dedup_fn = std::env::var("NYASH_RESOLVE_DEDUP_FN").ok().as_deref() == Some("1");
         let seam_dbg = std::env::var("NYASH_RESOLVE_SEAM_DEBUG").ok().as_deref() == Some("1");
@@ -176,6 +226,14 @@ pub fn strip_using_and_register(
                     prelude.push_str(&inlined);
                     prelude.push_str("\n");
                     crate::runner::modes::common_util::resolve::seam::log_inlined_tail(&key, &inlined, seam_dbg);
+                } else if key.starts_with("builtin:") {
+                    // Handle built-in namespaces like builtin:nyashstd
+                    let builtin_content = generate_builtin_namespace_content(&key);
+                    prelude.push_str(&builtin_content);
+                    prelude.push_str("\n");
+                    if verbose {
+                        eprintln!("[using] loaded builtin namespace: {}", key);
+                    }
                 } else if verbose {
                     eprintln!("[using] warn: could not read {}", p.display());
                 }

src/runner/pipeline.rs

@@ -242,7 +242,16 @@ pub(super) fn resolve_using_target(
         crate::runner::box_index::cache_put(&key, out.clone());
         return Ok(out);
     }
-    // 2) build candidate list: relative then using-paths
+    // 2) Special handling for built-in namespaces
+    if tgt == "nyashstd" {
+        let out = "builtin:nyashstd".to_string();
+        if trace {
+            crate::runner::trace::log(format!("[using/resolve] builtin '{}' -> '{}'", tgt, out));
+        }
+        crate::runner::box_index::cache_put(&key, out.clone());
+        return Ok(out);
+    }
+    // 3) build candidate list: relative then using-paths
     let rel = tgt.replace('.', "/") + ".nyash";
     let mut cand: Vec<String> = Vec::new();
     if let Some(dir) = context_dir {