bug(mq): embedded NATS installs its own SIGINT handler — races main shutdown, panics, and zeroes e2e coverage #287

What happens

natsserver.Server.Start() installs a process-wide signal handler unless opts.NoSigs is set (nats-server v2.14.0 server.go:948 → signal.go). Its SIGINT path is:

case syscall.SIGINT:
    s.Shutdown()
    s.WaitForShutdown()
    os.Exit(0)

WaveHouse's own graceful shutdown (cmd/wavehouse/main.go:421) runs concurrently on the same SIGINT, and main.go:227 already defers embeddedMQ.Close() → server.Shutdown(). So every SIGINT is a three-way race:

1. NATS handler wins → os.Exit(0) mid-drain — skips all of run()'s defers (dedup.Close, cache.Close, chConn.Close), truncating graceful shutdown. Exit 0, so nobody notices.
2. Main's path wins cleanly → correct shutdown.
3. Both interleave → double Server.Shutdown() → panic: close of nil channel in shutdownEventing (events.go:1907) → exit status 2.

Observed

Mode 3 hit a local make ci run today (tree = main + docs-only changes). The e2e orchestrator SIGINTs the instrumented binary to flush coverage; the panic exit skips the Go coverage atexit hooks, so:

2026/06/05 20:18:27 → SIGINT-ing WaveHouse (flushes coverage)...
2026/06/05 20:18:27   wavehouse exit: exit status 2
...
  e2e             0.0%  >=60  ✗      ← only covmeta written, no covcounters
==> Coverage gate FAILED: e2e

panic: close of nil channel

goroutine 89 [running]:
github.com/nats-io/nats-server/v2/server.(*Server).shutdownEventing(0x45fb95fad008)
	nats-server/v2@v2.14.0/server/events.go:1907 +0x134
github.com/nats-io/nats-server/v2/server.(*Server).Shutdown(0x45fb95fad008)
	nats-server/v2@v2.14.0/server/server.go:2569 +0x3c
github.com/nats-io/nats-server/v2/server.(*Server).handleSignals.func1()
	nats-server/v2@v2.14.0/server/signal.go:52 +0x318

Mode 1 has likely been silently truncating shutdowns all along (the Trapped "interrupt" signal NATS log line is visible in every e2e shutdown).

Fix

NoSigs: true in the embedded server options (internal/mq/embedded.go) — an embedded server must never own process signals; WaveHouse's shutdown path is the single owner and already calls Close().