The mesh's interface takes over the found tunnel's MTU

Carries MTU from the reported tunnel (mesh-host#28) through inventory,
the overlay graph's TakeOver, into the generated config's [Interface].
A tuned path keeps its MTU across the takeover instead of regressing to
1420 and hanging transfers no ping would reveal. Two emit tests; a
tunnel with no MTU writes no line.

internal/overlay/graph.go

@@ -50,6 +50,8 @@ type TakeOver struct {
 	Interface string
 	Unit      string
 	Config    string
+	// MTU is the found tunnel's, when it set one — emitted so a tuned path keeps its MTU.
+	MTU int
 	// Port is the port the found tunnel listened on. The mesh's interface must listen on it too,
 	// even on a node that is not dialable from the hub: a home node's LAN peers dial it there
 	// (novox/hq: a taken tunnel brings its port). Listening is "a peer dials me here"; it is not