[Snapshots] PK snapshot: Persisted cursor: store + migration + recorder + resume

[kvch]

What to build

Extend the Store interface + snapshot_requests migration to persist/read cursors; recorder writes cursors during snapshot; a fresh run resumes mid-table from the store.

Requirements

• Cursor persisted per (table, sub-range) during a run
• Process restart mid-table resumes from the persisted cursor

Depends on

• [Snapshots] PK snapshot: Persisted-cursor storage-shape decision #940 — Persisted-cursor storage-shape decision
• [Snapshots] PK snapshot: In-run reconnect/resume (cursor + backoff) #939 — In-run reconnect/resume (cursor + backoff)