Step 8: Benchmark

### Step 8: Benchmark

**Create** `bench/ull_bench.exs`
- Operations: update, update_many (1k, 100k), merge, estimate
- Pure vs Rust columns (same pattern as `hll_bench.exs`)
- Benchee with `warmup: 1, time: 3`, JSON output to `bench/output/ull_bench.json`

**Modify** `mix.exs` -- add `"run bench/ull_bench.exs"` to bench alias

