Improve query perf guards and memory accounting

[johnny-emp]

Summary

• make table memory accounting more conservative and document the operational sizing caveat
• avoid unnecessary residual filtering/diagnostic allocation for exact indexed predicates
• add planner support and regression coverage for simple leftmost equality on composite indexes
• expand query predicate benchmark coverage for Eq, In, LIKE prefix, residual LIKE, partial indexes, and composite indexes
• reduce avoidable join row/key allocation churn

Benchmarks

• Clean origin/main index_equality_by_age median: 11.323 us
• This branch index_equality_by_age median: 9.4565 us
• Approx. 16.5% faster median on the measured path

Tests

• cargo test --lib query::executor::tests
• cargo test --lib query::planner::tests
• cargo test --lib mem_bytes_running_counter_matches_full_walk
• cargo test --lib lib_tests::kv_api
• cargo test --test query_integration
• cargo test --test security_read_policy --test security_properties --test security_boundaries
• cargo test --test benchmark_gate benchmark_index_predicate_shapes -- --ignored --test-threads=1