Stage 6 of the ABS-compat port. Lifts the plugin's abssocket package
into internal/audiobooks/abssocket/, mounted under /abs/socket.io/ on
silo's main HTTP listener. The plugin's standalone-listener
workaround is no longer needed because the host plugin-proxy WS-bridge
constraint disappears once the code runs in-process.
Redis pub/sub adapter behavior is preserved when REDIS_URL is set;
single-replica in-memory fan-out otherwise.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
