feat(moq-net,moq-cli): per-track stats + moq-cli --stats usage display

[kixelated]

Summary

Adds a live usage display to moq-cli and makes the moq-net stats per-track to back it. Run any transfer with --stats to see which tracks are being produced/consumed and a continuously rewritten upload/download table:

track                  rate        total    frames    groups
test.hang         24.3 KB/s      99.4 KB       129         6
  0.avc3          24.3 KB/s      99.2 KB       128         5
  catalog.json        0 B/s        162 B         1         1

The stats system was broadcast-scoped and only exposed via a published .stats broadcast. This makes it per-track and adds an in-process read API so a CLI can render it directly.

Broadcast totals stay O(1) to read. The existing broadcast-level Counters rollup is kept in sync on every bump, so reading a total is a single atomic load and never iterates the per-track map. The per-track maps are only walked when someone actually wants the per-track view (the snapshot task once per tick, or a snapshot() call).

moq-net

• Add TrackCounters + per-(side, tier) track maps on BroadcastEntry. The track guards (PublisherTrack / SubscriberTrack) now bump both the broadcast rollup and the track's own atomics.
• Add Stats::snapshot() -> StatsSnapshot (plus public BroadcastSnapshot / SideSnapshot / TrackCounts / SessionCounts) for in-process consumers. No broadcast subscription required.
• The published per-broadcast JSON gains an additive tracks object; the wire frame now reuses SideSnapshot rather than a duplicate struct. Tier::idx is now public since the snapshot arrays are tier-indexed.
• Dead tracks are GC'd in the snapshot task once their last subscription closes; their bytes remain counted in the rollup.

moq-cli

• --stats / --stats-interval flags on publish, subscribe, serve, accept.
• Builds an enabled aggregator (throwaway origin), attaches the tier handle via with_stats, and a render loop diffs successive snapshot()s to compute rates and repaints a per-track table on stderr (stdout stays clean for piped media). A process is single-direction, so the table just shows the active broadcast/track totals.

Docs

• doc/bin/relay/config.md: document the new tracks field in the stats JSON.
• doc/bin/cli.md: document --stats.

Cross-package sync

The moq-net wire/API row points at js/net + doc/concept. There's no JS stats aggregator to mirror; the per-broadcast stats JSON is a relay-internal feature (consumed by dashboards), and its format change is additive and documented in doc/bin/relay. No peer wire-protocol change.

Test plan

• moq-net unit tests pass (374), including 3 new ones: per-track rollup, closed-track GC keeps rollup, no-op aggregator snapshot is empty.
• cargo fmt + clippy clean (via nix); moq-relay / moq-native still build.
• Manual end-to-end: ffmpeg | moq-cli serve + moq-cli subscribe --stats over QUIC renders the per-track table with live rates.

(Written by Claude)