[multicast] default the multicast feature and wire SoftNPU end-to-end

[zeeshanlakhani]

This enables multicast by default so the packaged artifacts include multicast support without requiring explicit feature flags at build time, and the separate multicast-test CI job is now folded into packet-test. This also wires up the softnpu ASIC backend to support multicast end-to-end by translating DPD's sidecar.p4 table operations into sidecar-lite.p4's simplified P4 pipeline.

asic/SoftNPU:

• We replace the stubbed AsicMulticastOps implementation (which returned "OperationUnsupported") for group creation and port addition with in-memory group tracking via McGroupData, following the tofino_stub pattern. Group membership is used by the table translation layer to build port bitmaps for sidecar-lite's Replicate extern. - We map sidecar.p4 table names to sidecar-lite equivalents and translate action parameters where the designs differ for emulation.
• We now track live replication entries per group and rewrite them when members are added or removed. SoftNPU bakes multicast port bitmaps into replication table entries at write time, unlike Tofino, where membership lives in the hardware PRE.
• We implement a TODO -> Table::clear. The management protocol has no clear message, so we emulate one by dumping table state and removing entries individually, covering the tagged and untagged VLAN variants.
• A fix for management reads: accumulate UART responses until the newline terminator rather than assuming a single 1024-byte read, and size the UDS receive buffer past any expected dump, since datagram recv truncates silently.

dpd/p4:

• Fix egress multicast counter attribution. The external arm previously preceded the underlay tag check and matched on every encapsulated replica, leaving underlay_mcast_ctr unreachable. Underlay attribution now keys on the UNDERLAY and UNDERLAY_EXTERNAL tags, and the external arm is the residual case.
  dpd/mcast:
• We fix an issue with stale in-memory sources after update with absent source list(s).
• During external-group rollback, we now delete bitmap and replication entries only for internal groups; those entries are only extant for internal groups. External groups share the internal group's IDs, so deleting unconditionally would destroy the live internal group's entries.

swadm:

• This adds a read-only swadm multicast subcommand (aliased as mcast as well) for inspecting the multicast groups programmed on the switch, with an #[ignore]d integration test run by the packet-test CI job.

References and Deps:

• Point p4rs at zl/multicast for bitmap-based replication softnpu#183
• [multicast] dep updates propolis#1093
• [multicast] Bitmap-based replication and bit-slice assignment for softnpu/a4x2 p4#240
• [multicast] Bitmap-based multicast replication for sidecar-lite sidecar-lite#152
• tokio pinned to 1.52.1, oxide-tokio-rt 0.1.4 (matching Omicron)