feat(plugins): ArrayBox/MapBoxプラグイン実装とPhase 10.1計画

🎯 革命的発見: プラグインC ABI = JIT→EXE変換の統一基盤

## 主な変更点
- ArrayBoxプラグイン: get/set/push/size/is_empty実装
- MapBoxプラグイン: size/get/has実装（ROメソッドのみ）
- Phase 10.1ドキュメント: プラグインBox統一化計画
- デモファイル3種: プラグイン動作確認用

## 技術的詳細
- BID-FFI (Box ID Foreign Function Interface) 活用
- 既存のプラグインシステムでJIT/AOT統一可能
- スタティックリンクでオーバーヘッド解消
- "Everything is Box → Everything is Plugin → Everything is Executable"

## テスト済み
- array_plugin_demo.nyash: 基本動作確認 ✅
- array_plugin_set_demo.nyash: set操作確認 ✅
- map_plugin_ro_demo.nyash: RO操作確認 ✅

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

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

examples/array_plugin_demo.nyash

@@ -0,0 +1,22 @@
+// ArrayBox plugin demo
+// Requires: plugins/nyash-array-plugin built (release) and nyash.toml updated
+// Run:
+//   NYASH_CLI_VERBOSE=1 ./target/release/nyash --backend vm examples/array_plugin_demo.nyash
+
+static box Main {
+  main() {
+    local a, i
+    a = new ArrayBox()
+    // push integers 1..5
+    i = 1
+    loop (i <= 5) {
+      a.push(i)
+      i = i + 1
+    }
+    me.console.log("len=", a.length())
+    me.console.log("get(0)=", a.get(0))
+    me.console.log("get(4)=", a.get(4))
+    return a.length()
+  }
+}
+

examples/array_plugin_set_demo.nyash

@@ -0,0 +1,20 @@
+// ArrayBox plugin demo (set)
+// Build plugin:
+//   (cd plugins/nyash-array-plugin && cargo build --release)
+// Run (plugin host auto-loads from nyash.toml):
+//   NYASH_CLI_VERBOSE=1 ./target/release/nyash --backend vm examples/array_plugin_set_demo.nyash
+
+static box Main {
+  main() {
+    local a
+    a = new ArrayBox()
+    a.push(10)
+    a.push(20)
+    a.push(30)
+    me.console.log("before set, len=", a.length(), ", get(2)=", a.get(2))
+    a.set(2, 42)
+    me.console.log("after  set, len=", a.length(), ", get(2)=", a.get(2))
+    return a.get(2)
+  }
+}
+

examples/map_plugin_ro_demo.nyash

@@ -0,0 +1,20 @@
+// MapBox plugin demo (RO via plugin: size/get/has), set is VM path.
+// Build plugins:
+//   (cd plugins/nyash-map-plugin && cargo build --release)
+// Run:
+//   NYASH_CLI_VERBOSE=1 ./target/release/nyash --backend vm examples/map_plugin_ro_demo.nyash
+
+static box Main {
+  main() {
+    local m
+    m = new MapBox()
+    // mutating ops (VM path)
+    m.set(1, 100)
+    m.set(2, 200)
+    me.console.log("size=", m.size())
+    me.console.log("get(1)=", m.get(1))
+    me.console.log("has(2)=", m.has(2))
+    return m.get(2)
+  }
+}
+