Add native batch scanner acceleration

[Krablante]

Summary

• add Linux/GNU native batch prefilter for cold full scans with automatic Node fallback
• split JSONL discovery into a dedicated module and add scanner diagnostics
• update public-safe benchmark docs with public baseline and synthetic parity evidence

Validation

• npm run check
• npm test
• npm run test:node
• npm run test:native
• npm run smoke
• npm run portable:smoke
• npm pack --dry-run --json
• public synthetic fixtures: small/medium/large/huge/adversarial accuracy match
• public real profile: 27.80s -> 10.46s cold wall, 45.36s -> 16.82s CPU, 0.350GB -> 0.180GB RAM

No release or npm publish performed.