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/screen_health_check.go

@@ -3,6 +3,7 @@ package tui
 import (
 	"context"
 	"fmt"
+	"os/exec"
 	"strings"
 
 	tea "github.com/charmbracelet/bubbletea"
@@ -155,6 +156,64 @@ func (m model) hcRunFanStress() (tea.Model, tea.Cmd) {
 	return m, nil
 }
 
+// startGPUStressTest launches the GPU Platform Stress Test and nvtop concurrently.
+// nvtop occupies the full terminal as a live chart; the stress test runs in background.
+func (m model) startGPUStressTest() (tea.Model, tea.Cmd) {
+	opts := hcFanStressOpts(m.hcMode, m.app)
+
+	ctx, cancel := context.WithCancel(context.Background())
+	m.gpuStressCancel = cancel
+	m.gpuStressAborted = false
+	m.screen = screenGPUStressRunning
+	m.nvidiaSATCursor = 0
+
+	stressCmd := func() tea.Msg {
+		result, err := m.app.RunFanStressTestResult(ctx, opts)
+		return gpuStressDoneMsg{title: result.Title, body: result.Body, err: err}
+	}
+
+	nvtopPath, lookErr := exec.LookPath("nvtop")
+	if lookErr != nil {
+		return m, stressCmd
+	}
+
+	return m, tea.Batch(
+		stressCmd,
+		tea.ExecProcess(exec.Command(nvtopPath), func(_ error) tea.Msg {
+			return nvtopClosedMsg{}
+		}),
+	)
+}
+
+// updateGPUStressRunning handles keys on the GPU stress running screen.
+func (m model) updateGPUStressRunning(msg tea.KeyMsg) (tea.Model, tea.Cmd) {
+	switch msg.String() {
+	case "o", "O":
+		nvtopPath, err := exec.LookPath("nvtop")
+		if err != nil {
+			return m, nil
+		}
+		return m, tea.ExecProcess(exec.Command(nvtopPath), func(_ error) tea.Msg {
+			return nvtopClosedMsg{}
+		})
+	case "a", "A":
+		if m.gpuStressCancel != nil {
+			m.gpuStressCancel()
+			m.gpuStressCancel = nil
+		}
+		m.gpuStressAborted = true
+		m.screen = screenHealthCheck
+		m.cursor = 0
+	case "ctrl+c":
+		return m, tea.Quit
+	}
+	return m, nil
+}
+
+func renderGPUStressRunning() string {
+	return "GPU PLATFORM STRESS TEST\n\nTest is running...\n\n[o] Open nvtop  [a] Abort test  [ctrl+c] quit\n"
+}
+
 func (m model) hcRunAll() (tea.Model, tea.Cmd) {
 	for _, sel := range m.hcSel {
 		if sel {