Standardize boot interface selection and management across NIC types

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Boot interface management grew up around one assumption -- that every host has a DPU in DPU mode, and our automation just picks the DPU's host interface as the boot device. As we expand past that (a DPU running in NIC mode, a DPU that's present but unused while an integrated NIC does north/south, and zero-DPU machines per #870), "which NIC does this host boot from?" needs the same answer for every NIC type, and an operator needs a dependable way to say "use this one."

This epic tracks finishing that standardization. The boot interface is the MachineBootInterface -- a NIC's MAC together with its Redfish EthernetInterface.Id -- and the goal is one selection rule, honored by every writer and reader, whether the chosen NIC is a DPU PF, a SuperNIC, or an integrated NIC.

Done so far (merged)

• feat: store the full boot interface for boot-device management #2261 -- record the full MachineBootInterface (MAC + Redfish id), MAC-first with id fallback.
• feat: and now enhance machine_interfaces with a full MachineBootInterface #2285 -- give machine_interfaces its own MachineBootInterface; the explored default and the managed value now speak the same type.
• feat: store every NIC interface ID, not just DPU interface IDs #2311 -- record the interface id for every host NIC, not just DPU PFs, so any NIC can be a boot interface.
• feat: make any host interface the primary, not just a DPU #2314 -- SetPrimaryInterface, the generic form of SetPrimaryDpu, so an operator can promote any interface (including on a zero-DPU host).
• feat: more consistently resolve the machine boot interface #2501 -- admin boot actions resolve from the machine_interfaces primary, via the same pick_boot_interface the machine-controller uses.
• feat: resolve the boot interface for machines awaiting their first lease #2528 -- admin boot actions resolve from predictions for a machine still waiting on its first DHCP lease.

What's left

• Honor a host's declared primary interface when picking its boot device #2657 -- honor a host's declared primary interface when picking its boot device.
• Resolve the boot interface from predictions in the machine-controller #2658 -- resolve the boot interface from predictions in the machine-controller (the other half of feat: resolve the boot interface for machines awaiting their first lease #2528).
• Retire nic_type; declare a host NIC's expected network segment (and type) directly #2659 -- retire nic_type; declare a host NIC's expected network segment (and type) directly.
• Honor a declared primary interface when computing the explored boot default #2662 -- honor a declared primary interface in the explored boot default (the Store A / site-explorer counterpart to Honor a host's declared primary interface when picking its boot device #2657).

Sequencing

PR 3 is independent (the segment axis, not boot). PR 1 and PR 2 can both start now, but PR 2's multi-NIC case depends on PR 1 (predictions have no primary flag, so the declared primary is the only disambiguator). If serial: PR 1 -> PR 2, with PR 3 anytime alongside.

Part of #870.