Story 4.2 — Per-BN auto-rollback to `weaver-prev.conf` on validation failure

**Epic:** #740 — Validation, rollback & observability

Design: [v4 design](https://github.com/automa-saga/traffic-shaper/blob/main/docs/bn-qos-multiclass-priority-design-v4-nft-priority.md) §11.

On a failed validation, auto-roll back the BN to its last-known-good nft state and alert; rollback is per-BN.

**Acceptance criteria:**
- Keep `weaver-prev.conf` (last-known-good) on disk; `nft -f weaver-prev.conf` restores prior state in one step.
- Re-render and re-apply on the next poll-loop tick after rollback.
- Rollback is scoped per-BN: a malformed update on one node never affects others; raise an alert.

Start with §11 of docs/bn-qos-multiclass-priority-design-v4-nft-priority.md and trace the poll-loop entry point that applies nft state. Determine where weaver-prev.conf is maintained and where validation failures are handled. Done means nft -f weaver-prev.conf restores only the failed BN, the next poll tick re-renders and reapplies it, and an alert is raised.

Written by the indexing model from the issue text.