fix: a source that has sent no CONFIG reads connecting, not connected

A stream that answered CONNECT but has sent no CONFIG has no signal list, so
nothing can be enabled and no DATA frame is owed. Calling that "connected, but
no data is arriving: another reader may hold the source" blamed a second reader
that does not exist, and the badge blinked green over it -- a working pipeline
is exactly what it did not have.

The glance word is `connecting` (ten characters, no wider than `event held`,
so the fixed-set layout rule still holds) and the badge takes a fourth, steady
yellow phase, both from the first frame rather than after the three-second
stall test. The stall notice itself now splits on `have_config`: the unicast
contention wording where a signal list exists, "trying to connect" where it
does not, with `[udpscope] connecting:` on stderr beside the old `starved:`.

src/status_text.hpp

@@ -11,9 +11,12 @@ namespace udpscope {
 /// two cannot disagree. No GTK here on purpose: this is the one part of the status bar that
 /// can be reasoned about as text.
 struct StatusSnapshot {
-  /// One word from the fixed set: "live", "paused", "armed", "capturing", "event held",
-  /// "disarmed". It is a word from a *fixed* set because the bar sits directly above the plot
-  /// and a field whose width tracked the stream would relayout it every frame.
+  /// One word from the fixed set: "connecting", "live", "paused", "armed", "capturing",
+  /// "event held", "disarmed". It is a word from a *fixed* set because the bar sits directly
+  /// above the plot and a field whose width tracked the stream would relayout it every frame --
+  /// so a word added here must be no wider than the widest already in it ("connecting" and
+  /// "event held" are both ten characters). "connecting" is the state before a CONFIG has
+  /// arrived, in which there is no signal list and so nothing any other word here can describe.
   const char* state_word = "live";
   /// The long prose the old single-line bar carried, kept for the detail string and the dump.
   std::string state_detail;