maglevt: show FQDN in header instead of config paths; v1.0.1

Replace the cfgPath field in the TUI header with the system's
fully-qualified hostname via gethostname + CNAME lookup, matching
what `hostname -f` produces.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

cmd/tester/model.go

@@ -75,7 +75,7 @@ type errEvent struct {
 // probeResultMsg handlers can update rolling/tally without copying
 // the whole model.
 type Model struct {
-	cfgPath string
+	host    string
 	vips    []*vipState
 	opts    probeOpts
 	startAt time.Time