Story 5.4a — Implement infra upgrade detection and set PendingInfraUpgrade

**Epic:** #502 — Network Upgrade Workflow (Execute Phase)

(Split from #540, part 1 of 3.) Detect whether an infrastructure upgrade is required and, if so,
transition the `NetworkUpgradeExecute` CR to `PendingInfraUpgrade`.

**Acceptance criteria:**
- Read currently-installed versions from `internal/state/software_state.go`.
- Parse the placed `infrastructure-versions.yaml` (`paths.InfraVersionsPath`) and compare its `host[]` and `cluster[]` entries against installed versions.
- If any declared version differs from installed, set CR status to `PendingInfraUpgrade`.
- If no version differs, skip infra upgrade entirely and continue the workflow (no phase change).
- Provisioner CLI/daemon version changes are detected here too but the actual self-upgrade is owned by epic #500.

**Depends on:** #539 (infra-versions placement).