Adds the test the project's contribution rules require for non-trivial logic
(AGENTS.md). Covers: posix passthrough (1 call, no sleep), Windows
retry-then-succeed, reraise after exhausting 5 attempts (no silent success on a
save path), no-retry on non-PermissionError, and the 50/100/200/400ms backoff
sequence. Production _safe_replace unchanged.
