[M-4] OverlayFs usage double-counts files across layers #418
Description
Finding M-4: Inaccurate OverlayFs Usage Accounting
Severity: MEDIUM
File: crates/bashkit/src/fs/overlay.rs:246-259
Threat ID: TM-DOS-036
Description
compute_usage() sums upper + lower layer usage without deducting overwritten or whited-out files. This makes usage() inaccurate and can cause premature limit rejections or, in combination with M-5, allow exceeding actual limits.
Recommended Fix
When computing combined usage, deduct entries that are overridden in the upper layer or present in the whiteout set.
Write a failing test first per AGENTS.md.