Make recovery writes atomic

[Pimpmuckl]

Summary

• replace mutable recovery-file writes with one same-directory temporary write plus rename helper
• preserve prior destinations and best-effort clean temporary files when replacement fails
• keep immutable session creation and artifact extraction writes unchanged

Validation

• plugin validator
• pnpm run build
• pnpm run lint
• pnpm test (278 passed)
• pnpm run format:check
• pnpm pack --dry-run
• git diff --check
• Review Suite fast: rvw_6d6cb83f (clean)