engine: throughput is opt-in in the run config; document the work

A 5-second run at 50 Mbps moves ~30 MB. On a metered connection that is the
user's money, and a measurement tool that spends it unasked is not one people
keep installed.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

echolot-app/core-engine/src/main/kotlin/app/echo_lot/engine/ServerMeasurement.kt

@@ -47,6 +47,14 @@ class ServerMeasurement(
          * it is a flag rather than an assumption.
          */
         val downstream: Boolean = true,
+        /**
+         * Throughput moves real data — a 5-second run at 50 Mbps is about 30 MB — so it is off
+         * unless asked for. On a metered mobile connection that is the user's money, and a
+         * measurement tool that spends it without being told to is not one people keep installed.
+         */
+        val throughput: Boolean = false,
+        @Suppress("unused") val throughputSeconds: Int = 5,
+        @Suppress("unused") val throughputKbps: Int = 50_000,
     )
 
     fun run(cfg: Config): MeasurementDocument {
@@ -89,6 +97,15 @@ class ServerMeasurement(
                 tests.addAll(dsTests)
                 allFindings.addAll(dsFindings)
             }
+
+            if (cfg.throughput && profile.supports("throughput")) {
+                val (tpTest, tpFindings) = ThroughputMeasurement(ids).run(
+                    cfg.credential, session.sessionId, control, ps, sessionRef = "sess-1",
+                    durationS = cfg.throughputSeconds, kbps = cfg.throughputKbps,
+                )
+                tests.add(tpTest)
+                allFindings.addAll(tpFindings)
+            }
         }
 
         control.deleteSession(cfg.credential, session.sessionId)