Slingshot v3.1.3 — entity list and index contracts

Highlights

• Makes entity pagination honor configured default sorts with deterministic cursor tie-breakers.
• Adds nested cross-backend list filters and rejects oversized limits instead of silently truncating results.
• Reconciles stable definition-derived indexes and enforces NULL-safe tenant uniqueness in PostgreSQL.
• Updates internal collection scans, consumer docs, and live backend conformance coverage.