[softnpu, mcast] wire up the softnpu backend for multicast table ops

[zeeshanlakhani]

Previously, SoftNPU didn't interact with multicast. mc_group_create and mc_port_add returned a stubbed OperationUnsupported, and port remove/destroy were no-ops.

With bitmap-based replication in p4rs (#240), SoftNPU can replicate off of port bitmaps carried in the replication table entry itself. On the Tofino, membership subscriptions live in the PRE and change independently of match-action state. In the SoftNPU work, membership is captured at write time, and a membership mutation means rewriting the affected replication entries (when triggered).

Includes:

• asic/SoftNPU:
  • Live replication entries are tracked per group and rewritten when members are added/removed. The replication tracking lock covers both the snapshot and table updates.
  • Source filter, mcast NAT (tagged and untagged), egress decap, and mcast src MAC tables now map onto their sidecar-lite tables respectively.
    • egress.asic_id_to_port maps ASIC port IDs to logical port numbers for indexing the decap bitmap (same as on Tofino).
  • TableRemove is sent before TableAdd because p4rs drops an add whose key already exists and keeps the older entry (action and params).
• dpd/mcast:
  • mcast::reset deletes each known group's entries before the table clears. SoftNPU's Table::clear is still a stub.
  • del_entries_where skips the stale-entry table sweep when the backend cannot list entries (e.g., in SoftNPU).
• softnpu-lib is pinned to zl/multicast @ softnpu#183, which pulls in p4#240. SoftNPU loads sidecar-lite#152 at runtime.