A handler cannot ask whether the client is still there #175

HttpResponse::sendable() is the only predicate a streaming handler has, and it answers four questions at once: the response is closed, or sealed by sendFile(), or has no stream ops, or the per-stream queue is full. Three of those are permanent and one is transient, so a caller cannot tell "yield and continue" from "stop".

isClosed() does not fill the gap — it reports that this code called end(), not that the peer left.

The consequence is in the field twice. Our own SSE example carried if (!$response->sendable()) break; // peer gone — stop early until #174; laravel-spawn wrapped the same call as Sse::connected() with the same wording; and YanGusik/laravel-spawn#60 truncated a proxied body at ~100 KB with that loop, under a declared Content-Length, which HTTP/2 then rejected.

Proposal

HttpResponse::isWritable(): bool — output is still possible: end() was not called, the response is not sealed by sendFile(), and the stream was not killed by the peer. Every input is a one-way latch, so a false answer stays false and the predicate is sound, unlike queue depth.

Backed by a new optional is_alive op in http_response_stream_ops_t (include/php_http_server.h:706). No backend needs new state: H2 has stream->peer_closed, the worker path has stream_credit_is_dead, H1 has the connection check h1_stream_append_chunk already makes, H3 the equivalent. A NULL slot answers from the response alone.

Part of the response body contract recorded in dev/PLAN.md.