[Snapshots] PK snapshot: schema preflight category + SnapshotKeyCheck

[kvch]

What to build

New CategorySchema + BuildSchemaChecks + --schema flag on checkCmd; SnapshotKeyCheck emits a blocking finding for pk-strategy tables lacking a usable key.

Requirements

• pgstream check --schema flags a keyless table with remediation in the message
• Passes when keys resolve; returns Go error only on connect/query failure

Depends on

• [Snapshots] PK snapshot: PK-resolution catalog helper #926 — PK-resolution catalog helper