Wait for every runtime discovery hears from, and name the ones it misses

The console gathered $SRV.INFO answers for a fixed 750 ms. The laptop's
runtime (48 modules, 341 endpoints, 164 kB) answers last every time: its
answer crosses to the broker on another machine and back, a median of
365 ms on a quiet link and 813 ms in one of 25 rounds measured. When it
missed the window the console said its modules ran nowhere ("nothing in
the mesh is called slack") or only on another machine.

Discovery now asks $SRV.PING alongside $SRV.INFO and waits, past the
window and up to 5 s, for every instance that answered PING. A runtime
that never sends what it serves, or answered before and not now, is
named in every answer that might concern it instead of the module being
called missing. An answer still too large after first-line descriptions
drops them, and says so in its metadata.

mesh_runtimes reports, per runtime, its machine, how long its answer
took, its size, its modules and tools, whether it was shortened and when
it was last heard, and the runtimes and machines not heard.

node-tools/internal/console/mcp.go

@@ -50,6 +50,8 @@ type Surface struct {
 	at    time.Time
 	idx   *index
 	idxAt time.Time
+	// runtimes is every runtime discovery has heard, as it last heard it (mesh_runtimes).
+	runtimes map[string]*runtimeSeen
 	// 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
@@ -300,7 +302,8 @@ func (s *Surface) instructions() string {
 	return "The tools of a Novox mesh, reached as " + s.who + ", found by address rather than listed " +
 		"whole (novox/hq ADR 0195). mesh_overview shows the mesh's seats and machines; mesh_machine one " +
 		"machine's seats and modules; mesh_search finds a tool by words; mesh_describe gives one tool's " +
-		"arguments; mesh_call calls it. " + grammar + " What may be called was fixed when this account " +
+		"arguments; mesh_call calls it; mesh_runtimes says which machines' runtimes answered discovery, how fast " +
+		"and how much — ask it when an address is said to be missing or on another machine. " + grammar + " What may be called was fixed when this account " +
 		"was issued, so a refusal means the account, not the tool."
 }