a request context on the ipc server, cancelled by Close (V-638)

serveConn dispatched under context.Background(), so a dispatch in flight
during shutdown could not be told to stop and closeGrace could only
abandon it. The server now carries a context, Close cancels it, and each
conn derives its own so nothing outlives the connection.

A zero-value Server built outside Listen falls back to Background; two
wiring tests do that.

Client.Close read c.conn with no lock while roundtrip re-dialed and
dropped it, which -race caught on the new test. The conn field now has
its own mutex, held only across a read or an assignment, so Close and
the watchdog reach the connection without queueing behind the call they
are interrupting.

internal/ipc/cancel_test.go

@@ -117,3 +117,54 @@ func TestClientDeadlineBoundsACall(t *testing.T) {
 		t.Errorf("Ping took %v, want the context deadline to bound it", elapsed)
 	}
 }
+
+// blockingAPI parks Presence until its context is cancelled and records what
+// cancelled it. Every other method is the unimplemented floor.
+type blockingAPI struct {
+	UnimplementedCoreAPI
+	entered chan struct{}
+	err     chan error
+}
+
+func (b *blockingAPI) Presence(ctx context.Context) (Presence, error) {
+	close(b.entered)
+	<-ctx.Done()
+	b.err <- ctx.Err()
+	return Presence{}, ctx.Err()
+}
+
+// serveConn dispatched under context.Background() until V-638, so Close could
+// only abandon a dispatch in flight and never tell it to stop.
+func TestServerCloseCancelsADispatchInFlight(t *testing.T) {
+	api := &blockingAPI{entered: make(chan struct{}), err: make(chan error, 1)}
+	srv, err := Listen(filepath.Join(t.TempDir(), "core.sock"), api)
+	if err != nil {
+		t.Fatalf("listen: %v", err)
+	}
+	served := make(chan struct{})
+	go func() { _ = srv.Serve(); close(served) }()
+
+	cli, err := Dial(srv.Path())
+	if err != nil {
+		t.Fatalf("dial: %v", err)
+	}
+	defer cli.Close()
+	go func() { _, _ = cli.Presence(context.Background()) }()
+
+	select {
+	case <-api.entered:
+	case <-time.After(5 * time.Second):
+		t.Fatal("the handler was never dispatched")
+	}
+
+	_ = srv.Close()
+	<-served
+	select {
+	case got := <-api.err:
+		if !errors.Is(got, context.Canceled) {
+			t.Errorf("handler saw %v, want context.Canceled", got)
+		}
+	case <-time.After(5 * time.Second):
+		t.Fatal("Close did not cancel the dispatch")
+	}
+}