Publish and consume MoQ traffic stats.

moq-net collects per-session traffic counters in a stats::Registry; this crate turns that registry into MoQ broadcasts and back:

• [Producer] drains a registry on an interval and publishes the counters as JSON tracks on an origin.
• [Consumer] subscribes to one published stats broadcast and yields typed frames, for aggregators, dashboards, and billing meters.
• [aggregate::Consumer] folds a whole group's per-node broadcasts into one merged view, so a downstream sees a project's total live traffic as if it came from a single node.

Wire format

A [Producer] publishes one broadcast per node at <prefix>/node/<node> (default prefix .stats), or one per group of leading broadcast-path segments at <prefix>/<group>/node/<node>; parse announce paths back with [parse_node_path]. Each [Tier] carries publisher.json, subscriber.json, and sessions.json tracks of cumulative [Traffic] and [Presence] counters, plus .json.z siblings encoded with [moq_json::snapshot]; compute names with [traffic_track] / [sessions_track]. The full contract (paths, tracks, both encodings, and counter semantics) is at https://doc.moq.dev/concept/stats.