Say what the serial number messages do

The README and the comments said the pool "goes out" and named it in
places where the reader cannot tell what is sent. They now name the
message and what it holds: the highest number taken on each band.

One of them was also wrong. The pool has not been sent on a reset since
the reset started counting from the log, and the README still said it was.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015NZkFxHde1iifAQemFTHxf

src/Nonemm.Session/SerialNumberPool.cs

@@ -36,8 +36,8 @@ public sealed class SerialNumberPool
     }
 
     /// A number another station has taken. False when it clashes with what is
-    /// held here — the other station is behind, and the caller answers with the
-    /// whole pool so it can catch up.
+    /// held here: the other station is behind, and the caller answers it with
+    /// `Snapshot`.
     public bool Note(int number, Band? band)
     {
         int at = N1mmBandIndex.Of(band);