Improve cache management (issue #8)

[virtualian]

Summary

• Move cache to ~/.npm-scanner/ (persistent across reboots)
• Add 200MB size limit with automatic eviction of oldest files
• Change expiration from 24h to 3 days
• Minimize cached data to only needed fields (~97% smaller: 600 bytes vs 17KB)
• Add package list cache for faster counting (mtime invalidation)
• Add npm-scanner.sh cache command (--status, --clear, --clear-expired)
• Display cache stats after scans

Test plan

• Cache invalidation when node_modules changes
• Scoped packages (@scope/pkg) handled correctly
• Size limit eviction removes oldest files
• 3-day expiration logic works
• --clear-expired command works
• Multiple node_modules directories cached separately
• Cross-platform stat commands (macOS/Linux fallback)
• End-to-end performance (~14x faster counting on cached runs)

Closes #8