[zeeshanlakhani]

This is the sixth (and last, maybe) part in a series of recut PRs split from #675. After pt5, we had ddm peers exchanging multicast subscriptions, but the imports never made their way to the switch, and mgd couldn't originate groups into ddm yet. This work handles both.

Includes:

• A multicast member sweep in ddmd (ddm/src/mcast.rs), which is tasked with programming imported routes into the DPD underlay group members (rear ports for now, geared toward mcast v1).
  • This sweep runs every 10s and can get woken up early by the db whenever an (origin, nexthop, interface) 3-tuple import is added or removed (via watch notify).
  • A read-modify-write per group per pass. The write passes back the group tag from the read. Nexus still owns group create/delete, and External members are not updated.
  • Only members programmed by this ddmd process (since startup) get removed (same as unicast + tunnel routes).
  • QSFP "Underlay" members are skipped for now (TODO), as replicas would cycle between racks without RPF or a designated forwarder.
• Multicast imports are structured and keyed per (nexthop, ifname) link, and remove_nexthop_{mcast_}routes take the interface. Before, the same link-local nexthop on two interfaces collapsed into one entry, where losing either link caused a drop.
• A new ddm admin API version (MULTICAST_INTERFACES), where get_multicast_groups returns ImportedMulticastRoute with the interface name.
• Transits only redistribute imports that are new or changed (including over path/metric modifications).
• mg-common::tfport kills a TODO and introduces a parser for tfport<kind><port>_<link>[.vlan] names, replacing the prefix/suffix string handling we dealt with previously.
• mg-lower MRIB origination uses a separate thread (from unicast/tunnels) that diffs loc_mrib against ddm's originated groups and advertises/withdraws accordingly.
  • This is a watcher-driven task with a 1s fallback resync, like unicast.
• testing:
  • We introduce a dual-sidecar topology, with a server with one link to each sidecar (e.g., sl0 -> sidecar a, sl1 -> sidecar b), importing from both, dropping and restoring sl1, and performing a ddmd restart.
  • Multicast phases have been added to the trio, quartet (failover), and sextet integrations.
• deps
  • For dual-sidecar testing ~ SoftNPU purposes, we are pinned against [softnpu, mcast] wire up the softnpu backend for multicast table ops dendrite#405 for dendrite.