wip(stage1): StringHelpers.skip_ws receiver捏造問題の応急処置

- 問題: StaticCompiler method呼び出し時にreceiver ValueIdが捏造され未定義エラー
- 応急処置: string_helpers.hako で src を明示的に文字列化 (""+src)
- 再現ケース: apps/tests/stage1_skip_ws_repro.hako 追加
- エラー: use of undefined value ValueId(28) in ParserBox.length(receiver=ValueId(28))

根本修正は次のcommitで実施:
- src/mir/builder/ssa/local.rs - receiver origin/type伝播強化
- Phase 25.1: Stage-1 bridge receiver bug (workaround)

apps/tests/stage1_skip_ws_repro.hako

@@ -0,0 +1,5 @@
+static box Main {
+  main() {
+    return StringHelpers.skip_ws("  a", 0)
+  }
+}