refactor(moq-net): unify stats counters on Gauge/Usage, drop the guard zoo

[kixelated]

Stacked on #1873 (base stats-in-model; retarget to dev once that merges).

Follow-up cleanup to the stats aggregator, applying the "just counters" shape from #1873 to the half it didn't touch. No behaviour change — same counters, same emitted .stats wire format.

1. One Gauge primitive for every lifecycle counter

The lifecycle counters were six loose AtomicU64s, and every guard's Drop re-derived the subtle "load closed (Acquire) before open (Relaxed)" snapshot ordering by hand. Now there's one Gauge { open, closed } with open() (RAII) / snapshot(), and the ordering lives in exactly one place (Gauge::snapshot). Counters becomes three Arc<Gauge> (announced / subscriptions / broadcasts) plus the shared Arc<Usage> from #1873; SessionCounters is just a Gauge.

2. Collapse the guard zoo

One generic Guard RAII token does the close-on-drop. The publisher/subscriber guard pairs collapse into side-parameterized types:

• PublisherStats + SubscriberStats → BroadcastGuard (announced)
• PublisherTrack + SubscriberTrack → TrackGuard (subscriptions + a Meter over the payload counters)

Each delegates its close to the embedded Guard — six hand-written Drop impls and four guard types become two. (SessionStats and the refcounting SessionBroadcasts/BroadcastSubscription keep their roles but now bump a Gauge.)

3. Finish the payload migration

TrackGuard no longer carries frame()/bytes()/group() — the only remaining caller was the lite-publisher fetch path, which now bumps the shared Usage through a Meter (fetch reads random-access frames the model doesn't auto-meter). So every payload count flows through Usage/Meter, and the guards are pure lifecycle.

Net

stats.rs is ~120 lines shorter; the tricky memory ordering is centralized and pinned by one source-level test (the two old ordering tests merge into one).

Testing

moq-net 390 lib + moq-native 62 e2e pass (lite 01–03 + IETF 14–18, fetch, compression); clippy clean on changed files. (moq-relay/workspace still blocked locally by the pre-existing iroh-relay requires rustc 1.91.)

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