feat: using system完全実装＋旧スモークテストアーカイブ完了

✅ using nyashstd完全動作（ChatGPT実装）
- builtin:nyashstd自動解決
- 環境変数不要でデフォルト有効
- console.log等の基本機能完備

✅ Fixture plugin追加（テスト用最小構成）
✅ v2スモークテスト構造への移行
✅ 旧tools/test/smoke/削除（100+ファイル）

🤖 Generated with Claude Code
Co-Authored-By: Claude <noreply@anthropic.com>

src/using/spec.rs

@@ -0,0 +1,42 @@
+//! Using specification models (skeleton)
+
+#[derive(Debug, Clone)]
+pub enum UsingTarget {
+    /// Logical package name (to be resolved via nyash.toml)
+    Package(String),
+    /// Source file path (absolute or relative)
+    SourcePath(String),
+    /// Dynamic library path (plugin)
+    DylibPath(String),
+}
+
+#[derive(Debug, Clone)]
+pub struct UsingSpec {
+    pub target: UsingTarget,
+    pub alias: Option<String>,
+    pub expose: Option<Vec<String>>, // planned
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum PackageKind {
+    Package,
+    Dylib,
+}
+
+impl PackageKind {
+    pub fn from_str(s: &str) -> Self {
+        match s {
+            "dylib" => PackageKind::Dylib,
+            _ => PackageKind::Package,
+        }
+    }
+}
+
+#[derive(Debug, Clone)]
+pub struct UsingPackage {
+    pub kind: PackageKind,
+    pub path: String,
+    pub main: Option<String>,
+    pub bid: Option<String>,
+}
+