The opt-in long-lived notification stream: server accepts a filter, acks first, routes only opted-in types, tags with subscriptionId; client demux helper for #2894 to wire in.

What's in it

• Server: accept subscriptions/listen with a filter, send the ack first, route only opted-in notification types, tag each with subscriptionId.
• A way for the existing notification publishers (list_changed, resources/updated) to feed the listen stream.
• Client demux helper (open/maintain stream, demux by id, re-subscribe after a stdio reconnect) — wired in by New Client (all-versions, low-level + high-level) #2894.

Conformance

• The sep-2575-server-* subscription checks inside server-stateless run (not skip) and pass
• Together with Backwards compatibility & version negotiation, New HTTP serving path (modern + dual-era), Capabilities API + server/discover handler, Context object (layered, per-request): server-stateless

Dependencies

• Depends on: New HTTP serving path (modern + dual-era) #2893

References

• Subscriptions
• SEP-2575