Phase 11.8/12: MIR Core-13 roadmap, Nyash ABI design, async/await enhancements with TaskGroupBox foundation

Major additions:
- Phase 11.8 MIR cleanup specification (Core-15→14→13 roadmap)
- Nyash ABI unified design document (3×u64 structure)
- TaskGroupBox foundation with cancelAll/joinAll methods
- Enhanced async/await with checkpoint auto-insertion
- Structured concurrency preparation (parent-child task relationships)

Documentation:
- docs/development/roadmap/phases/phase-11.8_mir_cleanup/: Complete Core-13 path
- docs/development/roadmap/phases/phase-12/NYASH-ABI-DESIGN.md: Unified ABI spec
- Updated Phase 12 README with AOT/JIT explanation for script performance
- Added async_task_system/ design docs

Implementation progress:
- FutureBox spawn tracking with weak/strong reference management
- VM checkpoint integration before/after await
- LLVM backend async support preparation
- Verifier rules for await-checkpoint enforcement
- Result<T,E> normalization for timeout/cancellation

Technical insights:
- MIR as 'atomic instructions', Box as 'molecules' philosophy
- 'Everything is Box' enables full-stack with minimal instructions
- Unified BoxCall for array/plugin/async operations future consolidation

Next steps:
- Complete TaskGroupBox implementation
- Migrate from global to scoped task management
- Implement LIFO cleanup on scope exit
- Continue Core-13 instruction consolidation

🚀 'From 15 atoms to infinite programs: The Nyash Box Theory'

src/boxes/mod.rs

@@ -84,6 +84,7 @@ pub mod debug_config_box;
 pub mod gc_config_box;
 pub mod aot_config_box;
 pub mod aot_compiler_box;
+pub mod task_group_box;
 
 // Web専用Box群（ブラウザ環境でのみ利用可能）
 #[cfg(target_arch = "wasm32")]
@@ -122,6 +123,7 @@ pub use jit_strict_box::JitStrictBox;
 pub use jit_hostcall_registry_box::JitHostcallRegistryBox;
 pub use aot_config_box::AotConfigBox;
 pub use aot_compiler_box::AotCompilerBox;
+pub use task_group_box::TaskGroupBox;
 
 // EguiBoxの再エクスポート（非WASM環境のみ）
 #[cfg(all(feature = "gui", not(target_arch = "wasm32")))]
@@ -158,7 +160,7 @@ pub use null_box::{NullBox, null};
 pub use array::ArrayBox;
 pub use buffer::BufferBox;
 pub use file::FileBox;
-pub use future::{NyashFutureBox, FutureBox};
+pub use future::{NyashFutureBox, FutureBox, FutureWeak};
 pub use json::JSONBox;
 pub use result::{NyashResultBox, ResultBox};
 pub use http::HttpClientBox;