Let the config choose protocol 2 or 3, and fall back for one session only.

The default stays v2. A reflector that refuses that version and names the other gets one session of it, then the station announces the configured protocol again.

Co-authored-by: Cursor <cursoragent@cursor.com>

station/protocol.go

@@ -194,6 +194,23 @@ func parseVersion(b []byte) (uint16, uint16, error) {
 }
 
 var atMostVersion = regexp.MustCompile(`^Unsupported protocol version \d+\.\d+\. May be at most (\d+)\.(\d+)\.$`)
+var atLeastVersion = regexp.MustCompile(`^Unsupported protocol version \d+\.\d+\. Must be at least (\d+)\.(\d+)\.$`)
+
+// wantsOther reports that the reflector refused the announced major and named the other one.
+func wantsOther(announced uint16, msg string) bool {
+	if announced >= 3 {
+		return fallback20(msg)
+	}
+	return fallback30(msg)
+}
+
+// acceptsOther reports that a downgrade names the protocol this station did not announce.
+func acceptsOther(announced, gotMajor, gotMinor uint16) bool {
+	if announced >= 3 {
+		return gotMajor == 2
+	}
+	return gotMajor == 3 && gotMinor == 0
+}
 
 // fallback20 reports that the reflector's maximum is at least 2.0 and below 3.0.
 func fallback20(msg string) bool {
@@ -209,6 +226,20 @@ func fallback20(msg string) bool {
 	return !versionBelow(major, minor, 2, 0) && versionBelow(major, minor, 3, 0)
 }
 
+// fallback30 reports that the reflector's minimum is exactly 3.0.
+func fallback30(msg string) bool {
+	m := atLeastVersion.FindStringSubmatch(msg)
+	if m == nil {
+		return false
+	}
+	major, err1 := strconv.Atoi(m[1])
+	minor, err2 := strconv.Atoi(m[2])
+	if err1 != nil || err2 != nil {
+		return false
+	}
+	return major == 3 && minor == 0
+}
+
 func versionBelow(major, minor, maj, min int) bool {
 	if major != maj {
 		return major < maj