Route a call key naming its machine after '@', and refuse a malformed key before publishing

The lease write asked seat:node-service-manager.stop@novox-be through Conn.Call, which published
the '@' inside the subject: nobody served it and the call timed out after 30 s with nothing said
(issue 774 notes 84-87). Every key now parses in one place, calladdress: Call routes '@node' as
CallOn does, and stdio Ask refuses a key the runtime could not route, before anything is sent.

Claude-Session: https://claude.ai/code/session_01NcbgfJ9Gum47zkFKWRaovL

go/stdio_test.go

@@ -5,6 +5,7 @@ import (
 	"encoding/json"
 	"errors"
 	"io"
+	"strings"
 	"sync"
 	"testing"
 	"time"
@@ -160,6 +161,12 @@ func TestABundleSubscribesAndAsksThroughTheRuntime(t *testing.T) {
 	if !TopicMatches("a.#", "a.b.c") || TopicMatches("a.*", "a.b.c") || !TopicMatches("#", "x") {
 		t.Error("topic matching differs from the TypeScript SDK's")
 	}
+	// Issue 774 notes 84-87: a malformed key is refused before the runtime is asked.
+	for _, key := range []string{"seat:node-service-manager.stop@novox@g14", "seat:a.*", "postgres.list@", "postgres.list@a.b"} {
+		if _, err := Ask(key, nil); err == nil || strings.Contains(err.Error(), "Serve") {
+			t.Errorf("Ask(%q) = %v, want the key refused", key, err)
+		}
+	}
 }
 
 // A subscribe the runtime refused leaves no handler behind: asked again until the runtime takes it, a