README test count stale; document createPropertyIndex; add scale benchmark #14

• README says npm test # 294 tests passing. Current state: 2 suites fail (OOM / timeout), masked by CI.
• createPropertyIndex is the largest measured win and is under-documented: where({email}) at 100k nodes went from 22.4ms (full scan) to 0.022ms, with EXPLAIN QUERY PLAN showing SEARCH n USING INDEX idx_merge_Person_email (type=? AND <expr>=?).
• Add the scale benchmark script to benchmarks/ so these numbers are reproducible.

Benchmark report: 2026-11-02, Apple M2 Pro, Node 22, at a7f9ea7.