[Snapshots] PK snapshot: Percentile boundaries (incl. TABLESAMPLE)

[kvch]

What to build

Compute balanced sub-range boundaries via percentile_disc, refined over a TABLESAMPLE subset on large tables to bound cost; leading-column split for composite keys.

Requirements

• Boundaries balance row counts under skew
• Split-query cost bounded on a large-table benchmark

Depends on

• [Snapshots] PK snapshot: Sub-range execution model #937 — Sub-range execution model