feat: MIR TypeOp/WeakRef/Barrier PoC implementation

- Add TypeOpKind, WeakRefOp, BarrierOp enums for unified instructions
- Implement TypeOp instruction combining TypeCheck/Cast
- Implement WeakRef instruction combining WeakNew/WeakLoad
- Implement Barrier instruction combining BarrierRead/BarrierWrite
- Update VM to handle new unified instructions
- Update MIR printer for new instruction formats
- Add feature flags mir_typeop_poc and mir_refbarrier_unify_poc
- Maintain backward compatibility with legacy instructions

This is Phase 8.5 MIR instruction diet PoC implementation.

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

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

docs/reference/architecture/dynamic-plugin-flow.md

@@ -113,3 +113,6 @@ Nyash Source ──▶ MIR (Builder)
 - `docs/reference/architecture/mir-to-vm-mapping.md`
 - `docs/reference/architecture/mir-26-instruction-diet.md`
 
+See also
+- `docs/examples/http_result_patterns.md` - HTTPのResult挙動（unreachable/404/500）のE2E例
+- `docs/VM_README.md` - VM統計とプラグイン周りの既知制約