Parallelize syng path-step repair

[ekg]

Summary

• rebuild .syng.pstep from existing GBWT paths in parallel by encoding per-path chunks and reducing them in path order
• add per-path progress logging, serial fallback, and optional explicit pstep output path
• save repaired pstep files atomically

Tests

• cargo test parallel_path_step --lib -- --nocapture
• cargo test syng --lib -- --nocapture
• cargo test --test test_syng_integration -- --nocapture
• CLI smoke: build a tiny syng, remove .pstep, repair it in parallel, and query through the repaired index