fix(core): serve a connection that arrives while the listeners are paused, and count the accepts the cap refuses (#359)

[EdmondDantes]

Closes #359. Needs true-async/php-async#306 merged first: this repository's CI builds php-async main, where h1/062–064 read 503, so the PR stays a draft until then.

• Tests:
  • h1/062: a third client during a pause at setMaxConnections(2).
  • h1/063: three connections at once past a cap of two.
  • h1/064: setMaxConnections(1), a resume that pauses again while delivering.
  • Each reads 503 on php-async main and passes 10 of 10 with fix(websocket): drain before closing an overflowed peer #306.
  • The full phpt suite passes 516 of 516 with fix(websocket): drain before closing an overflowed peer #306 (25 skipped).
• Counters in getTelemetry()/getStats():
  • accepts_refused_at_cap_total: the hard-cap safety net's 503s; a pause that holds keeps it at 0, and 062–064 assert that.
  • accept_errors_total: accept failures that were dropped silently.
  • telemetry/009 lists both.
• Comments: the backpressure note and http_server_pause_listeners said the REUSEPORT hash excludes a paused worker. It does not: the kernel keeps sending that worker its share, which waits for it.
• Plan: stage S1 in dev/PLAN.md. The design (pause by not accepting, over a uv_poll loop) went through a Critic and a Sage, and the php-async diff through a Critic.

[github-actions]

Coverage

Total lines: 83.75% → 83.79% (+0.04 pp)

File	Baseline	Current	Δ	Touched
src/core/http_connection.c	83.80%	83.64%	-0.15 pp
src/core/http_connection_tls.c	76.01%	76.26%	+0.25 pp
src/http3/http3_callbacks.c	86.69%	86.28%	-0.42 pp
src/http3/http3_listener.c	76.76%	78.41%	+1.65 pp
src/http3/http3_packet.c	85.22%	90.43%	+5.22 pp
src/http_server_class.c	75.99%	75.99%	-0.00 pp	●