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/view.go

@@ -78,6 +78,8 @@ func (m model) View() string {
 			body = renderNvidiaSATSetup(m)
 		case screenNvidiaSATRunning:
 			body = renderNvidiaSATRunning()
+		case screenGPUStressRunning:
+			body = renderGPUStressRunning()
 		case screenOutput:
 			body = fmt.Sprintf("%s\n\n%s\n\n[enter/esc] back  [ctrl+c] quit\n", m.title, strings.TrimSpace(m.body))
 		default: