Tenant-blind PostgreSQL admission engine.
This module owns the complete legacy claim plan: ready and expired selection, class progress and preference, claim/steal/poison mutation, row decoding, selection statistics, and the established claim telemetry.
The engine claims only under the legacy admission mode. Startup
configuration is last-boot-wins: it normalizes the persisted admission
mode to legacy when a previous configuration left it in another mode,
so a rebooted instance always claims, while dispatchers still running
another engine fail closed.