fix(EC-1816): validate envInt returns positive value

Reject zero and negative values for EC_STRESS_COMPONENTS and
EC_STRESS_WORKERS to fail fast instead of producing meaningless
benchmark results.

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

Author: dheerajodha

benchmark/stress/stress.go

@@ -55,6 +55,9 @@ func envInt(name string, fallback int) int {
 	if err != nil {
 		panic(fmt.Sprintf("invalid %s value %q: %v", name, v, err))
 	}
+	if n < 1 {
+		panic(fmt.Sprintf("%s must be >= 1, got %d", name, n))
+	}
 	return n
 }