Phase 10.10: GC Switchable Runtime & Unified Debug System 実装完了

Phase 10.10の主要実装:
- GcConfigBox: GC設定の実行時制御（counting/trace/barrier_strict）
- DebugConfigBox: デバッグ設定の統一管理（JIT events/stats/dump/dot）
- メソッドディスパッチ: system_methods.rsで両Boxのメソッド実装
- CountingGC動作確認: write_barriers正常カウント（VM実行時）

技術的詳細:
- BoxCore/BoxBase統一アーキテクチャを活用
- setFlag/getFlag/apply/summaryメソッドで統一API提供
- 環境変数経由でVM/JITランタイムと連携
- GcConfigBox.apply()は次回実行から有効（ランタイム作成前に環境変数参照）

テスト済み:
- examples/gc_counting_demo.nyash: CountingGCの動作確認
- write_barriers=3でArray.push/set, Map.setを正しくカウント
- NYASH_GC_TRACE=1でGC統計出力確認

Box-First哲学の体現: 設定も制御も観測もすべてBox！

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

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

examples/gc_counting_demo.nyash

@@ -0,0 +1,34 @@
+// GC Counting demo (VM path) — verifies CountingGc counters and barrier sites
+// Run:
+//   ./target/release/nyash --backend vm examples/gc_counting_demo.nyash
+// Expect (with trace): [GC] counters: safepoints>0 read_barriers>=0 write_barriers>=0
+
+static box Main {
+  main() {
+    // Turn on Counting GC + trace via Box-First path
+    local G
+    G = new GcConfigBox()
+    G = G.setFlag("counting", true)
+    G = G.setFlag("trace", true)
+    G.apply()
+
+    // Make sure we don't engage JIT path for this demo (VM baseline)
+    // If needed, you can set NYASH_JIT_EXEC=0 in the environment.
+
+    // Perform some mutating ops to hit write barriers on the VM path
+    local A, M
+    A = new ArrayBox()
+    A.push(1)
+    A.set(0, 2)
+
+    M = new MapBox()
+    M.set("k", "v")
+
+    // Simple read ops (should register read barriers on some implementations)
+    print(A.length())
+    print(M.size())
+
+    return "done"
+  }
+}
+