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>

Cargo.toml

@@ -20,6 +20,9 @@ dynamic-file = []
 wasm-backend = ["dep:wasmtime", "dep:wabt"]
 # プラグイン機構の有効化（ネイティブ環境のみ推奨）
 plugins = ["dep:libloading"]
+# MIR instruction diet PoC flags (scaffolding only; off by default)
+mir_typeop_poc = []
+mir_refbarrier_unify_poc = []
 # Note: LLVM feature requires inkwell dependency and LLVM development libraries
 # llvm = ["dep:inkwell"]