[multicast, refactor]: add ddm-peers omdb view, shift member writes to mg-lower

[zeeshanlakhani]

This PR stacks atop #10070 and inherits the multicast-to-physical (M2P) underlay forwarding and VMM-keyed instance subscription endpoints.

Under RFD 488 (incoming changes), and akin to unicast tunnel routes, mg-lower and ddmd derive underlay members directly from DDM peer subscriptions, so Nexus no longer resolves live sled-to-switch-port from inventory or DDM peers. The DDM client, instead, becomes a read-only peer view, and the reconciler's member->"Joined" path is a pure DB CAS plus best-effort sled-agent propagation, decoupled from MGS, the switch zone, and DPD member updates.

Includes:

• Adds the omdb nexus multicast ddm-peers view backed by the read-only DDM client, with a new internal-dns resolver helper (lookup_all_socket_v6_by_target) so per-switch clients correlate sockets back to their SRV targets
• Adds reconciler config knobs to MulticastGroupReconcilerConfig: group_concurrency_limit, member_concurrency_limit, and orphan_grace_secs (grace before an orphaned empty "Creating" group is reaped), wired through the example, SMF, and test configs
• Shifts underlay member writes to mg-lower, trimming the reconciler's member state machine and switch-zone coordination accordingly
• Splits the multicast dataplane client into focused modules: mrib.rs for per-pass MRIB route reconciliation and switch_zone.rs for the MGD/DDM switch-zone clients, leaving dataplane.rs to DPD group operations
• Adds schema migration multicast-member-origin (schema version 267): a new multicast_group_member_origin enum (static / igmp_snooped) and a membership_origin column on multicast group members defaulting to static. Groundwork for the RFD 488 static and IGMP/MLD paths, distinguishing which rows a future soft-state reaper may expire
• Aligns the simulated sled-agent's set_mcast_fwd with real OPTE's additive next-hop semantics, and reads back through the list_mcast_fwd API in the multi-sled test
  • Also, the sled-agent multicast membership API was renamed instance_join/leave_multicast_group → vmm_join/leave_multicast_group (for sim + client/HTTP surface).
• Corrects stale reconciler, switch-zone, and API-version docs throughout
• Documents the test boundary: coverage stops at the Omicron-owned edge, while
  DDM exchange and DPD member programming are maghemite/dendrite's domain
• Cleans up the now-dead inventory-based test scaffolding