Keep what discovery heard when a round hears nothing, and fetch only changed announcements (issue 554)

A round in which no PING reached the laptop's console within the 750 ms window
was taken as the mesh's answer that nothing is there, and replaced the index:
every seat and module call was refused for five seconds at a time. Discovery
now waits up to its patience for a first PING, a round that heard nothing is
not kept, a runtime not heard this round is resolved from what it announced
last (said with its age), and a round that misses runtimes is logged once.
A PING carries a digest of what the instance serves, so an unchanged
announcement is not pulled again every round (some 880 kB to the laptop).

node-tools/internal/console/mcp.go

@@ -52,6 +52,8 @@ type Surface struct {
 	idxAt time.Time
 	// runtimes is every runtime discovery has heard, as it last heard it (mesh_runtimes).
 	runtimes map[string]*runtimeSeen
+	// roundSaid is what the last round logged was missing, so a round is logged once (novox/hq issue 554).
+	roundSaid string
 	// Flat announces the whole catalogue, as the console did before ADR 0195: for a person reading
 	// it or a client that wants it. Off by default; MESH_CONSOLE_FLAT=1 turns it on.
 	Flat bool