Public Flow claim path.

This module validates FLOW.CLAIM_DUE / reclaim options, chooses the hot due index keys, coordinates blocking waiters, and delegates the actual mutation to the shard/Raft path. Returned payload/value hydration is capped and explicit.

Performance boundary

Claiming is a primary hot path. Avoid new maps/lists in per-candidate loops, behaviours/protocol dispatch, or extra GenServer calls. Any structural change here should be benchmarked with queued DBOS-style workers and focused claim_due tests.