feat(moq-net)!: gate broadcast announcements on a liveness flag

[kixelated]

What

Separate a broadcast's existence (registered in the origin tree, reachable by a FETCH) from its liveness (advertised to announced() subscribers), and drive announcement off a new per-broadcast liveness flag.

Publishing now registers a broadcast offline: publish_broadcast / create_broadcast make it FETCH-routable but do not announce it. A producer calls set_live(true) to advertise it and set_live(false) to stop advertising while it stays routable.

Why

Today the only way to reach a broadcast is via an announcement, so "exists" and "advertised" are the same thing. We want to route FETCH requests to offline broadcasts eventually, which means announcements should signal liveness, not existence. This PR makes that split: existence lives in the origin tree (what request_broadcast / a FETCH resolves against), and the announce stream is gated on the liveness flag. It also removes the accidental "announce before the catalog exists" race for callers that populate then go live.

How

• broadcast::{Producer,Dynamic}::set_live(bool) / is_live() and Consumer::is_live() back a liveness flag on the broadcast.
• The origin registers existence synchronously on publish, then a small per-broadcast reconciler watches the flag: it announces a live active route and unannounces one that goes offline or closes. Existence stays in the tree throughout; a promoted backup announces only if it is live.
• Route selection among broadcasts at the same path (active vs backup, hop/cost ordering) is unchanged.

API impact

Additive everywhere; nothing renamed or removed:

Layer	New
rs/moq-net	broadcast::{Producer,Dynamic}::{set_live,is_live}; on broadcast::Consumer is_live + the async observer live_changed / poll_live
rs/moq-ffi	MoqBroadcastProducer::set_live
rs/libmoq	moq_publish_set_live (new moq.h symbol)
py / swift / go / js	set_live / setLive / SetLive / Broadcast.live + setLive

Breaking (behavioral, not signature): in rust and js, publishing no longer announces on its own — callers must set_live(true). Hence this targets dev. The FFI-based bindings keep behavior: their announce verb marks the broadcast live internally, so announce still means "discoverable now" (additive only there).

No wire change / no IETF draft update: announce already is the wire liveness signal. cpp/obs unaffected (the C ABI change is a new function).

Consumer observer: broadcast::Consumer exposes an async live_changed(last) -> Option<bool> / poll_live (mirroring closed/poll_closed; None on close), so a rust consumer can react to a broadcast going offline — parity with the js Consumer.live signal. The origin's own unannounce reconciler drives off this same observer.

Interim API note

set_live is a standalone flag for now. Once #2241 lands the mutable broadcast::Route { hops, cost }, live should fold in as a third field (Route { hops, cost, live }) reconciled through the same route observer, dropping this PR's separate liveness channel and reconcile_publish reconciler. live is a genuine third axis, not cost = ∞: cost orders announced routes, live=false means don't announce but stay FETCH-routable.

Tests

• moq-net: new test_liveness_gates_announcement (offline = registered-but-not-announced and still FETCH-routable; toggling live drives announce/unannounce; unpublish removes it). Existing origin/announce suite updated for offline-by-default.
• Verified green: moq-net, moq-native, moq-rtmp, moq-relay, moq-srt, moq-hls, and @moq/net (js, incl. a wire-level regression).
• cargo doc -D warnings (public items, CI-equivalent) clean.

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