test(unit): bats coverage gaps for compute_backoff_seconds and validate_path edge cases

gh issue create --title "test(unit): bats coverage gaps for compute_backoff_seconds and validate_path edge cases" --label "test,audit,priority:low" --body "## Finding

tests/unit/lib.bats (or similar) does not cover:
- compute_backoff_seconds with _cb_delay=0
- compute_backoff_seconds with negative input
- validate_path with non-ASCII characters (the validator's regex may behave unexpectedly with multi-byte input)
- validate_path with newline (\\n) in input
- extract_netrc_host with empty string

## Affected code

- tests/unit/lib.bats

## Reproduction

N/A — coverage gap.

## Suggested fix

Add the missing bats test cases.

## Source

F2 audit 2026-09-03 (tests audit). Reporter: subagent-pekodbj7.

## Related

- Part of EPIC #[epic-number]
- Closes: #NONE (no existing issue)
- Related: lib.sh LOW-2 (extract_netrc_host coverage)"