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/forms.go

@@ -1,7 +1,6 @@
 package tui
 
 import (
-	"context"
 	"time"
 
 	"bee/audit/internal/platform"
@@ -140,20 +139,7 @@ func (m model) updateConfirm(msg tea.KeyMsg) (tea.Model, tea.Cmd) {
 				pollSATProgress("gpu-amd", since),
 			)
 		case actionRunFanStress:
-			m.busyTitle = "GPU Platform Stress Test"
-			m.progressPrefix = "fan-stress"
-			m.progressSince = time.Now()
-			m.progressLines = nil
-			since := m.progressSince
-			opts := hcFanStressOpts(m.hcMode, m.app)
-			return m, tea.Batch(
-				func() tea.Msg {
-					ctx := context.Background()
-					result, err := m.app.RunFanStressTestResult(ctx, opts)
-					return resultMsg{title: result.Title, body: result.Body, err: err, back: screenHealthCheck}
-				},
-				pollSATProgress("fan-stress", since),
-			)
+			return m.startGPUStressTest()
 		}
 	case "ctrl+c":
 		return m, tea.Quit