Function open_tier 

pub fn open_tier(
    backend: &str,
    dir: &Path,
    num_slabs: u32,
    slots_per_slab: u32,
) -> Result<Box<dyn ExpertTier>>

2026-09-25: Open the tier named by backend:

• posix, uma: read the store in dir;
• rdma: RdmaTier over TCP to $METRALE_EXPERT_PEER;
• rdma-verbs: RdmaTier with one-sided RDMA READ from $METRALE_EXPERT_PEER.