Simplify power calibration: pure binary search, no telemetry guessing

Remove telemetry-guided initial candidate; use strict binary search
midpoint at every step. Clean and predictable convergence in O(log N)
attempts within the allowed power range [minLimitW, startingLimitW].

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

audit/internal/platform/benchmark.go

@@ -2476,9 +2476,6 @@ func runBenchmarkPowerCalibration(
 	// calibSearchTolerance is the binary-search convergence threshold in watts.
 	// When hi-lo ≤ this, the highest verified-stable limit (lo) is used.
 	const calibSearchTolerance = 10
-	// calibPreThrottleMarginW is subtracted from the telemetry-estimated
-	// pre-throttle power draw to produce a smarter initial search candidate.
-	const calibPreThrottleMarginW = 10
 	// dcgmResourceBusyMaxDelaySec caps the exponential back-off when DCGM
 	// returns DCGM_ST_IN_USE (exit 222). The sequence is 1 s, 2 s, 4 s, …
 	// doubling each retry until it would exceed the cap, at which point the
@@ -2717,20 +2714,8 @@ func runBenchmarkPowerCalibration(
 				break
 			}
 
-			// Compute the next candidate.
-			// For thermal throttle: use the pre-throttle power draw from telemetry
-			// as a smarter initial estimate instead of the binary midpoint — it
-			// lands much closer to the true limit on the first attempt.
-			nextLimitW := (lo + hi) / 2
-			if strings.Contains(throttleReason, "thermal") {
-				if onsetW := calibPreThrottlePowerW(perGPU); onsetW > 0 {
-					candidate := roundTo5W(int(math.Round(onsetW)) - calibPreThrottleMarginW)
-					if candidate > lo && candidate < hi {
-						nextLimitW = candidate
-					}
-				}
-			}
-			nextLimitW = roundTo5W(nextLimitW)
+			// Binary midpoint within the remaining search range.
+			nextLimitW := roundTo5W((lo + hi) / 2)
 			// Ensure the candidate is strictly inside the search range.
 			if nextLimitW <= lo {
 				nextLimitW = lo + calibSearchTolerance
@@ -2770,28 +2755,6 @@ func isDCGMResourceBusy(err error) bool {
 	return errors.As(err, &exitErr) && exitErr.ExitCode() == 222
 }
 
-// calibPreThrottlePowerW estimates the GPU power draw just before thermal
-// throttle onset by averaging the first quarter of telemetry rows. The early
-// samples capture the GPU at peak before clock/power reduction kicks in.
-func calibPreThrottlePowerW(rows []GPUMetricRow) float64 {
-	if len(rows) < 4 {
-		return 0
-	}
-	n := len(rows) / 4
-	var sum float64
-	var cnt int
-	for _, r := range rows[:n] {
-		if r.PowerW > 0 {
-			sum += r.PowerW
-			cnt++
-		}
-	}
-	if cnt == 0 {
-		return 0
-	}
-	return sum / float64(cnt)
-}
-
 // roundTo5W rounds w to the nearest 5 W boundary.
 func roundTo5W(w int) int {
 	return ((w + 2) / 5) * 5