YAMAHA Router Interface Helper

Supported scope: the LAN-interface command subset of the YAMAHA RTX / NVR series (Rev.14-era syntax), matching the existing YAMAHA Router Filter Generator's scope. `pp` (PPPoE) and `tunnel` interfaces are out of scope; only `lan1` through `lan8` and their VLAN sub-interfaces (`lanN/i`) are supported. YAMAHA router-advertisement and DHCPv6-client configuration is model- and firmware-revision-dependent, so this subset will not guess at it: only the IPv6 methods "static" and "none" are supported, and "auto" or "dhcp" are rejected. From a bounded addressing plan (a lan1–lan8 port, an IPv4 method of none/dhcp/static, an IPv6 method of none/static, at most one static address per address family in CIDR form, one gateway per address family, up to 6 resolvers, at most 1 search domain, an optional MTU, and an optional VLAN sub-interface index with an 802.1Q VLAN id) it renders `ip`/`ipv6` address, MTU, and default-route commands, a `vlan ... 802.1q vid=...` command when a VLAN is configured, and `dns server` / `dns domain` commands. This page never connects to a router, and the available interface names and options vary by model and firmware revision. The commands are text only until entered in administrator mode and saved with `save`; check them against the target model's command reference first. Two platform limits are enforced here: the address commands set the primary address, so at most one static address per family is accepted rather than repeating the command and losing an address; and because the MTU ceiling differs by model — jumbo-frame support is not general across the series — only the safe common range up to 1500 is generated, so the shared model's generic 65535 ceiling is rejected here.