NEW: opnsense-set-interface-v4 pair (+harness 53/53) -- IPv4 + WAN gateway via the vendor config model ...

Operator-ruled mechanism 2026-07-20 ('Build the v4 setter pair'). Mirrors
the proven v6 sibling: vendor Config singleton (never a config.xml push),
dry-by-default, interface-count guard, fresh read-back verify, sh -s for
the tcsh trap, kernel ground truth. Adds the gateway half (at most ONE
gateway_item, count-guarded) and MEASURES the interface's device from the
edge's own config instead of hardcoding lan->vtnet0 like the v6 sibling.
D-113 amendment RE-MEASURED on 26.7: interfaces/settings/get carries only
offload flags; interfaces/overview/list and interfaces/<if>/get both 404 --
so the amendment holds on two majors. Gauntlet 71 harnesses ALL GREEN.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KiUu1oqt76tWvV4vEC3NAr