test(websocket): skip the room retry tests on the shared listen fd (#329)

[EdmondDantes]

websocket/064–066 test the cross-worker retry queue and need a subscriber on each of two workers. The server does not promise which worker accepts (#240), and on the shared listen fd — the second pass of LINUX_X64_DEBUG_ZTS — it measurably does not spread: 065/066 failed there three times in two runs of #328, and locally 065 read no subscriber on a remote worker on 3 of 10, 5 of 20 and 2 of 20 runs, against 0 of 16 on SO_REUSEPORT. The measurements of the skew are in #329; its cause is not traced.

The three tests now skip on the shared fd, through _ws_spread_skipif.inc, which mirrors core/076's condition. The retry queue reads nothing about the listener and stays covered by the first pass. Decided with Edmond; the helper and the workflow are unchanged.

dev/PLAN.md: the decision with its numbers, and an open item found on the way — http_server_pause_listeners may not stop accepting on the socket, to be reproduced first.

Verified

With the hooks build of main: TRUE_ASYNC_SERVER_SHARED_LISTEN_FD=1 skips all three with the reason; without it each passes 10 of 10.

[github-actions]

Coverage

Total lines: 83.67% → 83.66% (-0.02 pp)

File	Baseline	Current	Δ	Touched
src/http3/http3_callbacks.c	86.28%	86.17%	-0.10 pp
src/http3/http3_listener.c	77.06%	76.91%	-0.15 pp
src/http_server_class.c	76.04%	75.97%	-0.08 pp