feat(audit): add platform thermal cycling stress test

Runs CPU (stressapptest) + GPU stress simultaneously across multiple
load/idle cycles with varying idle durations (120s/60s/30s) to detect
cooling systems that fail to recover under repeated load.

Presets: smoke (~5 min), acceptance (~25 min), overnight (~100 min).
Outputs metrics.csv + summary.txt with per-cycle throttle and fan
spindown analysis, packed as tar.gz.

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

audit/internal/app/app_test.go

@@ -203,6 +203,10 @@ func (f fakeSAT) RunFanStressTest(_ context.Context, _ string, _ platform.FanStr
 	return "", nil
 }
 
+func (f fakeSAT) RunPlatformStress(_ context.Context, _ string, _ platform.PlatformStressOptions, _ func(string)) (string, error) {
+	return "", nil
+}
+
 func (f fakeSAT) RunNCCLTests(_ context.Context, _ string, _ func(string)) (string, error) {
 	return "", nil
 }