feat: use native PG17+ logical slot failover; retire spock worker on PG18

[rasifr]

Backport of PR #409 (SPOC-74) from main, squashed into a single commit.

• On PG17+, all logical replication slots are created with the FAILOVER
  flag, enabling PostgreSQL's built-in slotsync worker to synchronize
  them to physical standbys automatically.
• On PG18+, spock_failover_slots background worker is not registered;
  the native slotsync worker fully replaces it.
• On PG17, spock's worker remains active but yields (skips its sync
  loop) when sync_replication_slots = on is set, preventing conflicts.
• Fix PG15/16 failover slot loss on promotion: reconnect with retry
  and guard against zero WAL flush LSN.
• ZODAN: upgrade wait_for_sync_event calls to 5-arg form with timeout
  result checking; raise EXCEPTION on timeout/failure.
• Add tests/tap/t/018_failover_slots.pl and schedule entry.
• Add docs/logical_slot_failover.md and configuring.md section.

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🟢 Metrics 19 complexity · 0 duplication

Metric	Results
Complexity	19
Duplication	0

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