Implement bounded staging-store reaper

[niteshpurohit]

Objective

Remove expired and abandoned staging state with bounded background work.

Acceptance Criteria

The reaper removes:

• expired published entries;
• abandoned/incomplete transactions;
• orphan blobs;
• orphan sidecars; and
• incompatible/corrupt state according to safe cleanup policy.

Additional requirements:

• bounded entries/bytes/time per sweep;
• pressure-aware scheduling;
• safe reader/reaper races;
• configurable normal TTL;
• hard maximum-age/backstop default of 24 hours.

Technical Considerations

• Reaper work is reconstructible maintenance and yields under data-plane pressure.