Add src/tools/test-heartbeat-tolerance.sh: automated bash script that
deploys a 3-node cluster and runs two SIGSTOP/SIGCONT scenarios:
Scenario A (1 × heartbeat_interval_ms pause): verifies no RDHUP fires on
standbys — liveness_poll_fn must not trigger failover for a short silence
within the tolerance window (Req 2.2).
Scenario B (heartbeat_timeout_ms + 500ms pause): verifies exactly one RDHUP
fires on each standby — the activity-based poll fires after sustained silence
and does not loop (Req 2.3, Req 5.4).
Add .kiro/specs/ha-heartbeat-liveness/manual-tests.md: step-by-step manual
procedure, expected log patterns, and results template for both scenarios.
Mark task 4.5 complete in tasks.md.
