MRIB: Multicast RIB implementation

[zeeshanlakhani]

Implements the Multicast Routing Information Base (MRIB) for multicast support. The MRIB follows a two-table architecture (mrib_in → mrib_loc) with RPF verification against the unicast RIB when sources are provided.

This PR includes:

• rdb/src/db.rs: modifications to accomodate MRIB implementation and persistence
• rdb/src/mrib/mod.rs: the core MRIB implementation with route storage and change notifications
• rdb/src/mrib/rpf.rs: RPF verification using poptrie for O(1) LPM lookups, with rate-limited rebuilds triggered on unicast RIB changes
• rdb/src/types.rs: Validated multicast address types with input validation
• mg-api/src/lib.rs: API v3 (VERSION_MULTICAST_SUPPORT) with new endpoints
• mgd/src/mrib_admin.rs: HTTP handlers bridging API to MRIB
• mgadm/src/mrib.rs: CLI for MRIB inspection and configuration

Note that Omicron is source of truth multicast overlay/underlay groups/addresses.

[taspelund]

Is there an omicron branch that goes alongside this?

The main reason I ask is because I want to make sure everything builds okay.

In my IPv6 work I noticed that there were some build failures due to circular dependencies in the rdb crate, so to break the dependency loop I ended up moving a bunch of the types that omicron imports into a new rdb_types crate. To get my omicron branch (not merged yet) to build, I had to update it to depend on rdb_types instead of rdb.

I'm not sure if omicron would require the same thing for your changes, but I want to make sure this isn't a problem here too.

[zeeshanlakhani]

Yep. I included that in Cargo.toml:

oximeter = { git = "https://github.com/oxidecomputer/omicron", branch = "zl/mcast-implicit-lifecycle" }
oximeter-producer = { git = "https://github.com/oxidecomputer/omicron", branch = "zl/mcast-implicit-lifecycle" }
oxnet = { version = "0.1.4", default-features = false, features = ["schemars", "serde"] }
omicron-common = { git = "https://github.com/oxidecomputer/omicron", branch = "zl/mcast-implicit-lifecycle" }

[zeeshanlakhani]

But, yeah, I'll check on the circular dependency.

[zeeshanlakhani]

Seems like it'll work out, but I'll double-check this once we get the updates back in to Omicron.

[taspelund]

Things are looking good overall. So far, I've been able to review everything except for rdb/src/db.rs and rdb/src/mrib/rpf.rs. I'll plan to pick up where I left off either tomorrow or Wednesday.