fix(security): batch 2 — issues #493-#497

[chaliy]

Summary

• security: brace expansion {N..M} has no upper bound (TM-DOS-041, TM-DOS-042) #493/security: brace expansion {N..M} has no upper bound on range size (TM-DOS-041, TM-DOS-042) #494 (TM-DOS-041/042): Cap brace expansion {N..M} ranges at 10K elements and total expansion at 100K strings — oversized ranges treated as literals
• security: VFS copy() skips limits on overwrite, rename() overwrites dirs (TM-DOS-047, TM-DOS-048) #495 (TM-DOS-047/048): VFS copy() now always enforces size limits (even on overwrite); rename() rejects file-over-directory per POSIX
• security: OverlayFs symlink() bypasses all file count limits (TM-DOS-045) #496 (TM-DOS-045): OverlayFs::symlink() validates paths and enforces file count limits; symlinks now count toward file_count in compute_usage()
• security: date builtin leaks real host time (TM-INF-018) #497 (TM-INF-018): date builtin uses configurable fixed epoch via BashBuilder::fixed_epoch() instead of real host clock

Test plan

• All 17 security audit regression tests pass (cargo test -p bashkit --test security_audit_pocs)
• Full test suite passes (cargo test --all-features — 1515+ tests)
• cargo fmt --check clean
• cargo clippy --all-targets --all-features -- -D warnings clean

Closes #493, closes #494, closes #495, closes #496, closes #497

https://claude.ai/code/session_01TTiLUJVtmMNAo1NC9aQTn1