azd update: rollback/recovery on failed update

## Problem
No backup of current binary before replacement. A broken update via `azd update` or auto-apply requires manual reinstall.

## Proposed
- Back up the current binary before replacement (e.g. `azd.bak`)
- On failure, restore from backup automatically
- For auto-apply (`ApplyStagedUpdate`), use atomic copy-to-temp + rename instead of direct overwrite

## Context
Raised in design review #6910 and PR #6942 code review.

Parent epic: #6721