Believe the relay about its own roster, including an empty one

Same as the iPhone. Yesterday's guard refused every empty list and stranded a
removed last contact on every other device; the relay says which kind of empty
it means now, so the rule is simply whether the relay is the one holding the
roster.

MessengerApi.roster() carries both halves; buddies() stays for callers that
only want the list.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

client/src/main/kotlin/io/flamenet/client/MessengerApi.kt

@@ -44,8 +44,31 @@ class MessengerApi(
         val read: Boolean,
     )
 
-    fun buddies(): List<Buddy> {
-        val list = get("buddies").optJSONArray("buddies") ?: JSONArray()
+    /**
+     * The buddy list, and whether the relay is still the one holding it.
+     *
+     * §8.9c. A relay that has stopped keeping a contact graph answers with an
+     * empty list, and so does an account that genuinely has no contacts. They
+     * are different things and only the relay can tell them apart, so it says
+     * which. Absent on an older relay, and absent reads as "still keeps one",
+     * because that is what an older relay does.
+     */
+    data class Roster(
+        val buddies: List<Buddy>,
+        val relayKeepsRoster: Boolean,
+    )
+
+    fun roster(): Roster {
+        val body = get("buddies")
+        return Roster(
+            buddies = parseBuddies(body.optJSONArray("buddies") ?: JSONArray()),
+            relayKeepsRoster = body.optBoolean("roster_on_server", true),
+        )
+    }
+
+    fun buddies(): List<Buddy> = roster().buddies
+
+    private fun parseBuddies(list: JSONArray): List<Buddy> {
         return (0 until list.length()).map {
             val b = list.getJSONObject(it)
             // `id`, `display_name`, `screen_name`, `presence`, this route does