feat(hotblocks): measure fork consensus duration

[mo4islona]

Summary

• measure elapsed time from the first upstream fork signal to the consensus decision
• expose hotblocks_ingest_fork_consensus_duration_seconds with bounded decision labels: majority, all_active, timeout
• add decision and consensus_duration_seconds to the existing fork consensus log
• cover the GAP-41 path where one faulty source out of three reaches consensus through the 2 second timeout

This PR adds observability only; it does not change the GAP-41 consensus mechanism.

Tests

• cargo test -p sqd-data-source
• cargo test -p sqd-hotblocks --test ct4_lagging_source
• cargo test -p sqd-hotblocks metrics::tests
• cargo clippy -p sqd-data-source --all-targets --no-deps with existing baseline lint allowances
• cargo clippy -p sqd-hotblocks --test ct4_lagging_source --no-deps