Reduce IO cost of session cache GC size accounting

[zzj3720]

Follow-up from PR #60 review.

getPathSizeBytes currently recursively lstat/readdir walks every file under cache candidates so dry-run can report exact bytes. That is useful for observability, but large DerivedData or node_modules trees can create avoidable IO every cleanup interval, especially while dry-run remains enabled.

Explore lower-cost approaches:

• use a platform du -sk helper where available, or another bounded approximation;
• cap scan budget and continue in later intervals;
• only compute exact size when disk waterline is close/under threshold;
• preserve structured logs with a clear exact/estimated byte indicator.

Related PR: #60