[client, proxy] IPv6 in-place apply + accept-loop hardening on netstack listeners

Two related fixes for the embedded netbird client and the per-account
inbound listeners that ride on its gVisor netstack.

client/internal/engine.go — replace hasIPv6Changed with reconcileIPv6:

  - First v6 assignment (current had no v6, conf carries one) is applied
    in place via WGIface.UpdateAddr instead of returning ErrResetConnection.
    Pre-fix, every embedded client whose account had IPv6 enabled would
    reset on its first NetworkMap sync — boot config has no v6, the sync
    introduces one, the engine tore itself down to "apply" it. That
    teardown destroys the gVisor netstack and orphans every listener
    bound on it, which is what made the proxy's per-account :80/:443
    silently stop accepting traffic.
  - v6 removed clears in place.
  - v6 swapped to a different non-empty value still resets (gVisor
    netstack can't safely swap its address at runtime).
  - Mutates e.config.WgAddr to match the applied state so subsequent
    PeerConfig comparisons are stable.

proxy/internal/tcp/accept.go (new) + proxy/inbound.go +
proxy/internal/tcp/router.go — harden the two Accept() loops on
netstack-backed listeners:

  - IsClosedListenerErr recognises net.ErrClosed AND gVisor's
    "endpoint is in invalid state" — the latter survives gonet's
    *net.OpError wrapping in a way errors.Is(.., net.ErrClosed) does
    not. Without this the loop spins CPU-hot after the underlying
    netstack is destroyed (peer rekey, embedded-client reset, account
    churn), emitting one log line per iteration.
  - AcceptBackoff implements the exponential backoff that
    net/http.Server.Serve uses on transient Accept errors: 5ms doubling
    up to 1s. Defence-in-depth so an unknown sticky error cannot burn
    a CPU core even if IsClosedListenerErr misses its signature.

proxy/internal/roundtrip/netbird.go — emit a single structured INFO
line summarising every embed.Options flag (account_id, service_id,
public_key, management_url, wg_port, block_inbound, block_lan_access,
disable_ipv6, no_userspace, presence of credentials) when each
per-account embedded client is created. Secrets reduced to a "present"
boolean — never logged verbatim. Diagnostic-only; no behavior change,
but it makes the "why is this embedded peer misbehaving" loop a single
log read instead of a code dive.

Tests (real listeners, scripted errors, no mocks of production code):
  - engine_reconcileipv6_test.go: 8 cases for every transition (first
    assignment, no change, removed, prefix-length changed, value
    changed, invalid bytes, UpdateAddr error) plus a updateConfig
    integration check that the fix actually fires on a v6-added
    PeerConfig.
  - accept_test.go: IsClosedListenerErr matrix + AcceptBackoff
    progression / cap / reset / cancel-during-wait / cancel-before-call.
  - router_test.go, inbound_test.go: scriptedAcceptListener +
    TestRouter_Serve_ExitsOnGVisorInvalidEndpoint and
    TestFeedRouterFromListener_ExitsOnGVisorInvalidEndpoint —
    regression guards that fail in 2 s if the loop ever spins.

proxy/inbound_test.go

@@ -533,3 +533,125 @@ MHcCAQEEIIrYSSNQFaA2Hwf1duRSxKtLYX5CB04fSeQ6tF1aY/PuoAoGCCqGSM49
 AwEHoUQDQgAEPR3tU2Fta9ktY+6P9G0cWO+0kETA6SFs38GecTyudlHz6xvCdz8q
 EKTcWGekdmdDPsHloRNtsiCa697B2O9IFA==
 -----END EC PRIVATE KEY-----`)
+
+// scriptedAcceptListener returns pre-scripted errors from Accept(). Used
+// to drive the feedRouterFromListener tests without binding a real
+// socket — the production code path is a netstack-backed listener that
+// returns gVisor's "endpoint is in invalid state" forever after its
+// endpoint is destroyed.
+type scriptedAcceptListener struct {
+	errs   chan error
+	closed chan struct{}
+}
+
+func newScriptedAcceptListener(errs ...error) *scriptedAcceptListener {
+	s := &scriptedAcceptListener{
+		errs:   make(chan error, len(errs)+1),
+		closed: make(chan struct{}),
+	}
+	for _, e := range errs {
+		s.errs <- e
+	}
+	return s
+}
+
+func (s *scriptedAcceptListener) Accept() (net.Conn, error) {
+	select {
+	case <-s.closed:
+		return nil, net.ErrClosed
+	case err := <-s.errs:
+		return nil, err
+	}
+}
+
+func (s *scriptedAcceptListener) Close() error {
+	select {
+	case <-s.closed:
+	default:
+		close(s.closed)
+	}
+	return nil
+}
+
+func (s *scriptedAcceptListener) Addr() net.Addr {
+	return &net.TCPAddr{IP: net.IPv4(127, 0, 0, 1), Port: 0}
+}
+
+// errSentinel carries a literal error message so tests can synthesise
+// the exact gVisor text without importing the netstack package.
+type errSentinel string
+
+func (e errSentinel) Error() string { return string(e) }
+
+// TestFeedRouterFromListener_ExitsOnGVisorInvalidEndpoint is the
+// regression guard for the inbound side of the tight-loop bug. The
+// per-account plain-HTTP feeder must recognise gVisor's "endpoint is in
+// invalid state" and exit, otherwise it pegs a CPU core and floods the
+// account-scoped log with the same accept error every iteration.
+func TestFeedRouterFromListener_ExitsOnGVisorInvalidEndpoint(t *testing.T) {
+	logger := log.StandardLogger()
+	addr := &net.TCPAddr{IP: net.IPv4(127, 0, 0, 1), Port: 80}
+	router := nbtcp.NewRouter(logger, nil, addr)
+
+	gvisorErr := &net.OpError{
+		Op:   "accept",
+		Net:  "tcp",
+		Addr: addr,
+		Err:  errSentinel("endpoint is in invalid state"),
+	}
+	ln := newScriptedAcceptListener(gvisorErr)
+	defer ln.Close()
+
+	done := make(chan struct{})
+	go func() {
+		defer close(done)
+		feedRouterFromListener(context.Background(), ln, router, logger, "acct-1")
+	}()
+
+	select {
+	case <-done:
+		// Expected: loop recognised the gVisor error and returned.
+	case <-time.After(2 * time.Second):
+		t.Fatal("feedRouterFromListener did not exit on gVisor 'endpoint is in invalid state' — accept loop is spinning")
+	}
+}
+
+// TestFeedRouterFromListener_BacksOffOnTransientError asserts the
+// defence-in-depth path: an unknown sticky Accept error must NOT cause
+// CPU spin. The loop backs off and exits cleanly when ctx is cancelled.
+func TestFeedRouterFromListener_BacksOffOnTransientError(t *testing.T) {
+	logger := log.StandardLogger()
+	addr := &net.TCPAddr{IP: net.IPv4(127, 0, 0, 1), Port: 80}
+	router := nbtcp.NewRouter(logger, nil, addr)
+
+	const transientCount = 5
+	errs := make([]error, transientCount)
+	for i := range errs {
+		errs[i] = errSentinel("transient: temporary network error")
+	}
+	ln := newScriptedAcceptListener(errs...)
+	defer ln.Close()
+
+	ctx, cancel := context.WithCancel(context.Background())
+	start := time.Now()
+	done := make(chan struct{})
+	go func() {
+		defer close(done)
+		feedRouterFromListener(ctx, ln, router, logger, "acct-1")
+	}()
+	time.AfterFunc(150*time.Millisecond, cancel)
+
+	select {
+	case <-done:
+		// Expected.
+	case <-time.After(2 * time.Second):
+		t.Fatal("feedRouterFromListener did not exit on ctx cancellation — backoff or exit path broken")
+	}
+
+	// Without backoff the 5 scripted errors would burn in microseconds.
+	// With backoff the first delay alone is 5ms, so the loop must take
+	// at least that long even though ctx fires at 150ms.
+	elapsed := time.Since(start)
+	assert.GreaterOrEqual(t, elapsed, 5*time.Millisecond,
+		"loop ran without backing off — would burn CPU in production")
+}