perf(executor): optimize ORDER BY + LIMIT with top-k

[aviralgarg05]

Summary

Optimize ORDER BY ... LIMIT N execution using top-k selection instead of full sort when safe.

Why

Current implementation sorts all candidate rows even when only a small prefix is needed.

Tasks

• Update execution logic in nexum_core/src/executor/mod.rs
• Use a bounded structure (or equivalent approach) for top-k ordering
• Add benchmark coverage in nexum_core/benches/executor_bench.rs

Acceptance Criteria

• Query results remain identical to full-sort behavior
• Benchmark shows reduced runtime for large datasets with small LIMIT
• Code is covered by regression tests