Phase 21.3 WIP: Hako Source Checker improvements - HC011/HC016/HC017 実装完了

主な変更:
- ✅ HC011 (dead methods) 実装・テスト緑
- ✅ HC016 (unused alias) 実装・テスト緑
- ✅ HC017 (non-ascii quotes) 実装完了
- 🔧 tokenizer/parser_core 強化（AST優先ルート）
- 🛡️ plugin_guard.rs 追加（stderr専用出力）
- 📋 テストインフラ整備（run_tests.sh改善）

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

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

lang/src/shared/common/string_helpers.hako

@@ -171,4 +171,24 @@ static box StringHelpers {
     }
     return -1
   }
+
+  // Split string by newline into ArrayBox (without relying on StringBox.split)
+  split_lines(s) {
+    local arr = new ArrayBox()
+    if s == null { return arr }
+    local n = s.length()
+    local last = 0
+    local i = 0
+    loop (i < n) {
+      local ch = s.substring(i, i+1)
+      if ch == "\n" {
+        arr.push(s.substring(last, i))
+        last = i + 1
+      }
+      i = i + 1
+    }
+    // push tail
+    if last <= n { arr.push(s.substring(last)) }
+    return arr
+  }
 }