fix(conversion): lower shared S@/S! to NVVM shared address space

[tetsuo-cpp]

Summary

• fix shared-memory lowering for S@/S! by using NVVM shared address-space pointers
• replace hardcoded pointer address space with NVVM::kSharedMemorySpace in ForthToMemRef
• tighten memory-ops conversion test to require !llvm.ptr<3> for shared load/store lowering
• this makes generated PTX use ld.shared/st.shared for tiled matmul shared buffers, avoiding illegal memory access

Test plan

• cmake --build build --target format
• cmake --build build --target check-warpforth
• Verified pipeline PTX for tiled matmul emits ld.shared.u64/st.shared.u64 for shared buffers