refactor(audit): rename Fan Stress Test → GPU Platform Stress Test

Update all user-facing strings in TUI and ActionResult title.
Internal identifiers (types, functions, file name) unchanged.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

audit/internal/tui/forms.go

@@ -140,7 +140,7 @@ func (m model) updateConfirm(msg tea.KeyMsg) (tea.Model, tea.Cmd) {
 				pollSATProgress("gpu-amd", since),
 			)
 		case actionRunFanStress:
-			m.busyTitle = "Fan Stress Test"
+			m.busyTitle = "GPU Platform Stress Test"
 			m.progressPrefix = "fan-stress"
 			m.progressSince = time.Now()
 			m.progressLines = nil