Stop teaching sendable() as a liveness check

[EdmondDantes]

Documentation only.

README.md showed if (!$response->sendable()) break; // peer gone — stop early inside the SSE loop. sendable() reports whether send() would park, so that line ends a stream at the first sign of backpressure and drops what it was about to write. It travelled: laravel-spawn wrapped it as Sse::connected() with the same wording, and YanGusik/laravel-spawn#60 truncated a proxied body at ~100 KB with the same loop. The guard is removed rather than replaced — sseEvent() waits for room by itself, and a departed peer arrives as HttpException 499.

The sendable() docblock now says what false does not mean. The write() docblock said "Write data to response body buffer" without the part that matters: nothing leaves before end(), which is the other half of #60.

The docs/USAGE.md section on the three body modes is deliberately not in this PR. It would describe names that the contract in dev/PLAN.md renames in the next step, and the reporter is writing his proxy recipe against the new ones.

[github-actions]

Coverage

Total lines: 82.26% → 82.24% (-0.03 pp)

File	Baseline	Current	Δ	Touched
src/core/http_connection.c	75.45%	75.26%	-0.19 pp
src/http1/http_parser.c	84.06%	83.68%	-0.38 pp
src/http3/http3_callbacks.c	84.09%	83.65%	-0.44 pp
src/http3/http3_static_response.c	75.11%	76.37%	+1.27 pp
src/websocket/php_websocket.c	78.44%	78.61%	+0.17 pp
src/websocket/ws_session.c	90.41%	90.04%	-0.37 pp