Investigate StreamWatch reconnect status race

Investigate the synthetic StreamWatch incident where stage-b reconnects successfully but becomes offline while the current connection continues sending accepted heartbeats. Scope and constraints: - Inspect repository code, tests, documented reproduction, and documented test logs/endpoints only. - Reproduce locally and compare remote synthetic logs when available. - Identify the likely cause and define a focused deterministic regression test. - Do not edit implementation, tests, or configuration; do not deploy. Acceptance criteria: - Evidence distinguishes observations from the causal hypothesis. - Cause is tied to specific source locations and contradicted alternatives are considered. - A regression test scenario and expected assertions are identified.