[Snapshots] PK snapshot: Snapshot-only consistency warning

[kvch]

What to build

Emit a prominent warning when strategy: pk runs under the snapshot command (no CDC to reconcile drift).

Requirements

• Warning logged on pgstream snapshot with strategy: pk
• No warning under run (CDC follows)

Depends on

• [Snapshots] PK snapshot: Walking skeleton: keyset snapshot + batch sizing #928 — Walking skeleton: keyset snapshot + batch sizing