feat(tui): GPU Platform Stress Test — live nvtop chart during test

Apply the same pattern as NVIDIA SAT: launch nvtop via tea.ExecProcess
so it occupies the full terminal as a live GPU chart (temp, power, fan,
utilisation lines) while the stress test runs in the background.

- Add screenGPUStressRunning screen + dedicated running/render handlers
- startGPUStressTest: tea.Batch(stress goroutine, tea.ExecProcess(nvtop))
- [o] reopen nvtop at any time; [a] abort (cancels context)
- Graceful degradation: test still runs if nvtop is not on PATH
- gpuStressDoneMsg routes result to screenOutput on completion

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

audit/internal/tui/update.go

@@ -108,6 +108,28 @@ func (m model) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
 		return m.handleNvidiaGPUsMsg(msg)
 	case nvtopClosedMsg:
 		return m, nil
+	case gpuStressDoneMsg:
+		if m.gpuStressAborted {
+			return m, nil
+		}
+		if m.gpuStressCancel != nil {
+			m.gpuStressCancel()
+			m.gpuStressCancel = nil
+		}
+		m.prevScreen = screenHealthCheck
+		m.screen = screenOutput
+		m.title = msg.title
+		if msg.err != nil {
+			body := strings.TrimSpace(msg.body)
+			if body == "" {
+				m.body = fmt.Sprintf("ERROR: %v", msg.err)
+			} else {
+				m.body = fmt.Sprintf("%s\n\nERROR: %v", body, msg.err)
+			}
+		} else {
+			m.body = msg.body
+		}
+		return m, m.refreshSnapshotCmd()
 	case nvidiaSATDoneMsg:
 		if m.nvidiaSATAborted {
 			return m, nil
@@ -152,6 +174,8 @@ func (m model) updateKey(msg tea.KeyMsg) (tea.Model, tea.Cmd) {
 		return m.updateNvidiaSATSetup(msg)
 	case screenNvidiaSATRunning:
 		return m.updateNvidiaSATRunning(msg)
+	case screenGPUStressRunning:
+		return m.updateGPUStressRunning(msg)
 	case screenExportTargets:
 		return m.updateMenu(msg, len(m.targets), m.handleExportTargetsMenu)
 	case screenInterfacePick: