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/backend/vm_boxcall.rs

@@ -262,6 +262,22 @@ impl VM {
             }
         }
 
+        // TaskGroupBox methods (scaffold → instance内の所有Futureに対して実行)
+        if box_value.as_any().downcast_ref::<crate::boxes::task_group_box::TaskGroupBox>().is_some() {
+            let mut owned = box_value;
+            if let Some(tg) = (&mut *owned).as_any_mut().downcast_mut::<crate::boxes::task_group_box::TaskGroupBox>() {
+                match method {
+                    "cancelAll" | "cancel_all" => { return Ok(tg.cancelAll()); }
+                    "joinAll" | "join_all" => {
+                        let ms = _args.get(0).map(|a| a.to_string_box().value.parse::<i64>().unwrap_or(2000));
+                        return Ok(tg.joinAll(ms));
+                    }
+                    _ => { return Ok(Box::new(VoidBox::new())); }
+                }
+            }
+            return Ok(Box::new(VoidBox::new()));
+        }
+
         // P2PBox methods (minimal)
         if let Some(p2p) = box_value.as_any().downcast_ref::<crate::boxes::p2p_box::P2PBox>() {
             match method {