[FEATURE] Add PrefixPolicy for safe prefix seek and bloom-based pruning

[feichai0017]

🚀 Is your feature request related to a problem?

Prefix scan behavior and bloom-based pruning are currently not governed by a unified policy. This makes correctness/performance tradeoffs less explicit and harder to tune per workload.

💡 Proposed Solution

Introduce a PrefixPolicy abstraction to control:

• exact-prefix seek behavior,
• safe prefix bloom/table pruning rules,
• fallback behavior when prefix assumptions are not guaranteed.

🎨 Alternatives Considered

1. Keep ad-hoc prefix handling spread across iterators/tables.
2. Expose only boolean switches.

A dedicated policy type is clearer and easier to evolve.

📝 Technical Details / Implementation Plan

• Define PrefixPolicy options and wire them into iterator/table selection.
• Ensure correctness-safe fallback path when prefix pruning cannot be guaranteed.
• Add benchmarks for prefix-heavy workloads to quantify pruning win.

🧐 Additional Context

This feature should prioritize correctness first, then optimize with bloom/table pruning where safe.