[multicast] M2P forwarding, OPTE port subscription, and sled-agent propagation

Complete the multicast data path by adding per-sled M2P (multicast-to-
physical) mapping, forwarding entry management, and OPTE port subscription
for multicast group members.

Sled-agent:
- Add `multicast_subscribe` / `multicast_unsubscribe` endpoints (API v29)
  that configure M2P, forwarding, and OPTE port subscription for a VMM
- OPTE port_manager gains set/clear operations for M2P and forwarding
- Port subscription cleanup on PortTicket release

Nexus:
- New `sled.rs` (MulticastSledClient) encapsulating all sled-agent
  multicast interactions: subscribe/unsubscribe, M2P/forwarding
  propagation and teardown
- Groups RPW propagates M2P and forwarding entries to all member sleds
  after DPD configuration, with convergent retry on failure
- Members RPW uses MemberReconcileCtx to thread shared reconciliation
  state. This handles subscribe on join, unsubscribe on leave, and
  re-subscribe on migration
- Dataplane client updated for bifurcated replication groups

Tests:
- Integration tests for M2P/forwarding/subscribe lifecycle
- Instance migration multicast re-convergence

Author: zeeshanlakhani

Cargo.lock

@@ -648,7 +648,7 @@ omicron-workspace-hack = "0.1.0"
 omicron-zone-package = "0.12.2"
 oxide-client = { path = "clients/oxide-client" }
 oxide-tokio-rt = "0.1.2"
-oxide-vpc = { git = "https://github.com/oxidecomputer/opte", rev = "e547d07b08c3f3d6c821c9eb7a958adcffce6e56", features = [ "api", "std" ] }
+oxide-vpc = { git = "https://github.com/oxidecomputer/opte", branch = "zl/filter-mcast-srcs", features = [ "api", "std" ] }
 oxlog = { path = "dev-tools/oxlog" }
 oxnet = "0.1.4"
 once_cell = "1.21.3"
@@ -657,7 +657,7 @@ openapiv3 = "2.2.0"
 # must match samael's crate!
 openssl = "0.10"
 openssl-sys = "0.9"
-opte-ioctl = { git = "https://github.com/oxidecomputer/opte", rev = "e547d07b08c3f3d6c821c9eb7a958adcffce6e56" }
+opte-ioctl = { git = "https://github.com/oxidecomputer/opte", branch = "zl/filter-mcast-srcs" }
 oso = "0.27"
 owo-colors = "4.2.2"
 oximeter = { path = "oximeter/oximeter" }