Previous logic caught 404 on heartbeat and re-registered, but the re-
registration failed silently if the tracker wasn't fully ready yet.
New approach: after any connection-refused streak, the heartbeat loop
switches to re-registration mode and keeps retrying until the tracker
accepts the registration (instead of sending heartbeats into a fresh
registry that doesn't know this node). Falls back to the 404 path for
the case where a node is purged without a full tracker restart.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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