Fix platform thermal cycling leaving GPU load running after test ends

bee-gpu-burn is a shell script that spawns bee-gpu-burn-worker children.
exec.CommandContext default cancel only kills the shell parent; the worker
processes survive and keep loading the GPU indefinitely.

Fix: set Setpgid=true and a custom Cancel that sends SIGKILL to the
entire process group (-pid), same pattern already used in runSATCommandCtx.
Applied to Nvidia, AMD, and CPU stress commands for consistency.

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

audit/internal/platform/platform_stress.go

@@ -392,6 +392,13 @@ func buildCPUStressCmd(ctx context.Context) (*exec.Cmd, error) {
 		cmdArgs = append(cmdArgs, "-M", strconv.Itoa(mb))
 	}
 	cmd := exec.CommandContext(ctx, path, cmdArgs...)
+	cmd.SysProcAttr = &syscall.SysProcAttr{Setpgid: true}
+	cmd.Cancel = func() error {
+		if cmd.Process != nil {
+			_ = syscall.Kill(-cmd.Process.Pid, syscall.SIGKILL)
+		}
+		return nil
+	}
 	cmd.Stdout = nil
 	cmd.Stderr = nil
 	if err := startLowPriorityCmd(cmd, 15); err != nil {
@@ -433,6 +440,13 @@ func buildAMDGPUStressCmd(ctx context.Context) *exec.Cmd {
 	cfgFile := "/tmp/bee-platform-gst.conf"
 	_ = os.WriteFile(cfgFile, []byte(cfg), 0644)
 	cmd := exec.CommandContext(ctx, rvsPath, "-c", cfgFile)
+	cmd.SysProcAttr = &syscall.SysProcAttr{Setpgid: true}
+	cmd.Cancel = func() error {
+		if cmd.Process != nil {
+			_ = syscall.Kill(-cmd.Process.Pid, syscall.SIGKILL)
+		}
+		return nil
+	}
 	cmd.Stdout = nil
 	cmd.Stderr = nil
 	_ = startLowPriorityCmd(cmd, 10)
@@ -448,6 +462,16 @@ func buildNvidiaGPUStressCmd(ctx context.Context) *exec.Cmd {
 		return nil
 	}
 	cmd := exec.CommandContext(ctx, path, "--seconds", "86400")
+	// bee-gpu-burn is a shell script that spawns bee-gpu-burn-worker children.
+	// Put the whole tree in its own process group so context cancellation kills
+	// all workers, not just the shell parent.
+	cmd.SysProcAttr = &syscall.SysProcAttr{Setpgid: true}
+	cmd.Cancel = func() error {
+		if cmd.Process != nil {
+			_ = syscall.Kill(-cmd.Process.Pid, syscall.SIGKILL)
+		}
+		return nil
+	}
 	cmd.Stdout = nil
 	cmd.Stderr = nil
 	_ = startLowPriorityCmd(cmd, 10)