feat(planner): Parquet-native adaptive filter with two-scan approach

Pre-scan uses projection (scalar + _key only, no vector column) with
filter pushdown for selectivity estimation. Low-selectivity path
(<= 5%) bypasses USearch and SQLite entirely — streams full Parquet
scan, evaluates filters inline, computes distances, and maintains a
top-k heap (ScoredRow) to return results directly.

High-selectivity path unchanged: HNSW filtered_search → SQLite fetch.

- Remove old brute-force path and heap_select_top_k helper
- Add parquet_native_execute() for the low-selectivity path
- Add ScoredRow struct with Ord impl for max-heap eviction

Author: anoop-narang