Step 6: Rust Backend Wiring

[thanos]

Step 6: Rust Backend Wiring

Modify lib/ex_data_sketch/nif.ex -- add 6 stubs (3 operations x 2 variants)

Modify lib/ex_data_sketch/backend/rust.ex:

• ull_new/1 and ull_update/3 delegate to Pure
• ull_update_many/3 dispatches to NIF with dirty threshold ull_update_many: 10_000
• ull_merge/3 dispatches to NIF (no dirty needed, same as HLL merge)
• ull_estimate/1 dispatches to NIF