Renew the engine placed by hand to the proved build, and take a hand-over from the controller's terminal (hq issue 356)

The file on PATH was the engine a person copied at installation, and every delivered engine ran from the versions directory: the operator typed hand-over and a build from before it answered "unexpected argument". A proved engine now copies itself over that file when the bytes differ (never a link). And the hand-over of a directory used as found (issue 339) gets its channel: the controller's terminal asks this node's engine on mesh.node.<node>.ask.hand-over; the engine judges every value — the ask's size and shape, who asked, the path as it states it, a directory and no link, through no link — and records it or refuses and records nothing. Every text the engine addresses to the operator names the nox line (ADR 0272); the engine's own subcommands are install-time and rescue, and its usage says so.

internal/link/queue.go

@@ -51,8 +51,11 @@ type Queue struct {
 	// Numbers keeps the report sequence and the refusals counted, across restarts and self-updates.
 	// Nil counts in memory, which is only right where nothing outlives the process — a test.
 	Numbers Numbers
-	Say     Announce
-	Timeout time.Duration
+	// HandOver judges and records a directory used as found handed to the mesh by the operator, asked by
+	// the controller over the link (novox/hq issue 356). Nil answers every such ask with a refusal.
+	HandOver HandOverFunc
+	Say      Announce
+	Timeout  time.Duration
 
 	once sync.Once
 	wake chan struct{}