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>

src/mir/mod.rs

@@ -18,7 +18,7 @@ pub mod value_id;
 pub mod effect;
 
 // Re-export main types for easy access
-pub use instruction::{MirInstruction, BinaryOp, CompareOp, UnaryOp, ConstValue, MirType};
+pub use instruction::{MirInstruction, BinaryOp, CompareOp, UnaryOp, ConstValue, MirType, TypeOpKind, WeakRefOp, BarrierOp};
 pub use instruction_v2::{MirInstructionV2, AtomicOrdering}; // New 25-instruction set
 pub use basic_block::{BasicBlock, BasicBlockId, BasicBlockIdGenerator};
 pub use function::{MirFunction, MirModule, FunctionSignature};