feat: Phase 10 reorganization + GC switchable runtime + VM improvements

## 📚 Documentation Updates
- Phase 10 reorganized with comprehensive README
  - Cranelift JIT as main implementation
  - NEW: Phase 10.4 GC Switchable Runtime (world's first\!)
  - Phase 10.5 Self-hosting (parallel)
  - Application migration tests
- Phase 11 created for LLVM AOT research (deferred)
  - Moved phase10_aot_scaffolding.md → Phase 11
  - Moved phase_10_x_llvm_backend_skeleton.md → Phase 11
- Master roadmap updated with GC runtime feature
- Ideas: GC switchable language concept documented

## 🚀 VM Implementation Progress (by ChatGPT5)
- src/backend/vm.rs: Enhanced VM execution
- src/backend/vm_instructions.rs: Instruction improvements
- src/runtime/type_meta.rs: NEW - Type metadata system
- src/boxes/buffer/mod.rs: Buffer optimizations
- src/runtime/mod.rs & plugin_ffi_common.rs: Runtime enhancements

## 🌟 Revolutionary Feature: GC Switchable Runtime
- Development mode: GC on (convenience)
- Production mode: GC off (performance)
- Technical feasibility confirmed by Codex GPT-5
- Implementation plan: After Cranelift JIT

## 📋 Phase 10 Structure
Phase 10.0: Cranelift JIT foundation
Phase 10.1-10.3: JIT implementation & optimization
Phase 10.4: GC Switchable Runtime ← NEW\!
Phase 10.5: Self-hosting (String/Array/Map in Nyash)
Phase 10.9: Application migration tests

🤖 ChatGPT5 says: Ready for Phase 10\! どきどきにゃ！

src/boxes/buffer/mod.rs

@@ -47,6 +47,11 @@ impl BufferBox {
         }
     }
 
+    /// バッファ内容をコピーしてVec<u8>として取得（FFIやエンコード向け）
+    pub fn to_vec(&self) -> Vec<u8> {
+        self.data.read().unwrap().clone()
+    }
+
     /// Rust向けヘルパー: バッファ長をusizeで取得（テスト用）
     pub fn len(&self) -> usize {
         self.data.read().unwrap().len()