SSE event subscriptions buffer without bound when the consumer stops draining #31923

Description

A client that opens /event or /global/event and stops draining leaks server memory. Each subscription buffers events in an unbounded queue, and the only cleanup is the response fiber ending, which requires a disconnect signal. A TCP connection stuck in CLOSE_WAIT never sends one, so the listener stays registered and the queue grows with every event the server emits.

The megathread has field measurements of this pattern: 24.5GB RSS with zombie CLOSE_WAIT connections, +434MB/30s growth at a stable zombie count (#20695 (comment)).

Two aggravators in current dev:

• the instance endpoint filters by directory after buffering, so a stalled subscriber accumulates all instances' events
• the global endpoint buffers through Stream.callback with the same exposure

Reproduction

Open an event endpoint, never read the body, publish events past any reasonable backlog size. The per-connection queue grows monotonically and the response stream never terminates. A deterministic regression test is included in the linked PR.

Version

Current dev. Matches the v1.4.3 behavior reported in the megathread; the Effect migration preserved the pattern (Queue.unbounded per connection in handlers/event.ts).

Part of #20695.