main: keep the cupsonline room list out of the KDF context

The room list is created by the exit when it starts and handed to
clients as --cupsonline-url, so the client derived the context from it
while the exit, which has no URL, used "http://#": different keys and
no handshake over real cups.online in --transports mode. It is skipped
like a missing URL now, which is also what upstream exits derive.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

logging_flags_test.go

@@ -46,6 +46,11 @@ func TestPickSessionContext(t *testing.T) {
 		{"highest-priority transport URL", "", "http://#", specs, "https://docs/doc"},
 		{"no URL anywhere keeps the old default", "", "http://#", specs[:1], "http://#"},
 		{"legacy single transport", "", "http://#", []transportSpec{{Type: "yandex", Priority: 100, URL: "http://#"}}, "http://#"},
+		{"cupsonline rooms are not a context", "", "http://#", []transportSpec{
+			{Type: "cupsonline", Priority: 100, URL: "WyIxYzE0NGQwZS1lMDQw"},
+			{Type: "yandex", Priority: 50, URL: "https://docs/doc"},
+		}, "https://docs/doc"},
+		{"cupsonline alone", "", "http://#", []transportSpec{{Type: "cupsonline", Priority: 100, URL: "WyIxYzE0NGQwZS1lMDQw"}}, "http://#"},
 	}
 	for _, c := range cases {
 		if got := pickSessionContext(c.explicit, c.url, c.specs); got != c.want {

main.go

@@ -144,9 +144,14 @@ func isNumber(s string) bool {
 //
 //	explicit      --session-context, if non-empty
 //	--url         globalURL, if set and not the placeholder
-//	transports    URL of the highest-priority transport that has one
+//	transports    URL of the highest-priority transport that has one,
+//	              cupsonline aside
 //	fallback      the placeholder "http://#"
 //
+// A cupsonline "URL" is the room list the exit creates when it starts and
+// prints for clients, so the exit cannot know it beforehand; letting it
+// into the context gave the two sides different keys.
+//
 // This is what the OpenFlux-Android client derives for a Session profile,
 // and the fallback is what older builds used whenever --url was unset, so a
 // node without any document URL (direct, oneme) keeps its old key.
@@ -160,7 +165,10 @@ func pickSessionContext(explicit, globalURL string, specs []transportSpec) strin
 	}
 	best := -1
 	for i, s := range specs {
-		if s.URL != "" && s.URL != placeholder && (best < 0 || s.Priority > specs[best].Priority) {
+		if s.Type == "cupsonline" || s.URL == "" || s.URL == placeholder {
+			continue
+		}
+		if best < 0 || s.Priority > specs[best].Priority {
 			best = i
 		}
 	}