perf(factory): cap firmware identification concurrency

[cfilipescu]

Summary

• Add a separate firmware-identification concurrency limit to MinerFactory scans.
• Let address workflows continue through TCP probing while firmware identification remains bounded.
• Preserve existing behavior when callers do not configure the new limit.

This supports the UMC scanner's staged scan: it schedules up to 512 address workflows while limiting firmware identification to 128 concurrent miners. On the test network, this avoided the long full-range retry and scanned one /22 in 10.1 seconds (251 devices) and the combined 12 /22 ranges in 87.8–97.3 seconds (5,366–5,407 devices). Results varied between consecutive runs, and the timings include device reads.

Validation

• cargo fmt -- --check
• cargo check -p asic-rs --all-features --locked
• cargo check --features engineering -p rig-runner-cli with this branch patched locally
• Release scan benchmarks on one /22 and the combined 12 /22 ranges