Fix O(n²) scaling in planner for nullable natural keys

Two optimizations that together provide 4x speedup for realistic workloads:

1. Canonical NK resolution (Step 21): When all new entities have uniform
   NK specificity (same non-null keys pattern), skip the O(n²) LATERAL
   join and use direct assignment. Reduces step from 60ms to 12ms.

2. Target rows filter (Step 9): Replace NULL-safe OR join pattern with
   partitioned equality joins. The old pattern caused O(n²) bitmap scans
   because PostgreSQL couldn't optimize the OR condition. New approach
   partitions by NULL pattern and uses simple IN clauses per partition.

Performance impact on test 112 (realistic SEED benchmark):
- SEED stat_for_unit (ES): 582 → 4,577 rows/sec (7.8x faster)
- Total SEED time: 60s → 15s (4x faster)

Root cause: Natural keys with nullable columns (e.g., stat_for_unit with
legal_unit_id XOR establishment_id) triggered worst-case query plans.

Author: jhf