F-31: JMRI heartbeat task deleted from outside, possibly mid-send #34

Severity: MEDIUM · Effort: Small · Batch: 2 · Status: Code done in #21 (68452a5), awaiting bench verification

Found in the second full review of the firmware. Recorded with the rest of the pass (F-19…F-44) in docs/REVIEW_REMEDIATION_PLAN_2.md, which is where its status is kept.

Description

JmriJsonClient::stopHeartbeat calls vTaskDelete on a task that may be inside
esp_websocket_client_send_text, holding the client's lock — the F-02 shape again. It is
called from both the WebSocket task and disconnect() with an unguarded handle, so the same
handle can be deleted twice. The task's stack is 2 KB with ESP_LOG in its path.

Plan

Cooperative shutdown with an exit signal, as F-02 did for the WiThrottle receive task, and a
3 KB stack pending F-33's measurement.

Acceptance Criteria

• The task is woken to exit by a notification, signals, and suspends itself;
  stopHeartbeat() deletes it only after that signal, under a mutex that also guards
  startHeartbeat().
• Stack raised to 3 KB.
• Bench: repeated JMRI JSON connect/disconnect (twenty cycles) with no hang and no heap
  growth.