[Snapshots] PK snapshot: Sub-range execution model

[kvch]

What to build

Given key-space boundaries, run table_workers parallel keyset walkers, each over its [lo,hi) sub-range, with exact, non-overlapping coverage.

Requirements

• N workers cover every row exactly once (incl. composite keys)
• Boundary rows are neither duplicated nor skipped

Depends on

• [Snapshots] PK snapshot: Non-integer & composite keys #929 — Non-integer & composite keys