app: probe the server this device is enrolled with, not ours

The canary-DNS zone and the STUN host were compiled in as c.echo-lot.app
and fmr-1.echo-lot.app, so every copy of the app measured against this
particular deployment whatever server its owner had enrolled with. On
someone else's install those two tests describe our infrastructure and
report the result as a fact about their network.

The zone comes from the server's own profile, which has advertised
canary_zone all along — the app simply never read it. It is cached in
settings because the canary probe runs at device tier, before anything
has contacted the control plane, and a probe that had to make a call
first would fail on exactly the networks worth measuring. The STUN host
is derived from the configured server URL rather than stored, since a
second copy of the server's name goes stale the moment someone
re-enrolls elsewhere.

With no server configured both now report SKIPPED. StunProbe previously
would have reported FAILED on a blank host, which reads as a finding
about the network when the truth is that no packet was ever sent — the
same conflation between "measured nothing" and "measured a fault" that
the ICMPv6 finding had.

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

echolot-app/core-probe/src/main/kotlin/app/echo_lot/probe/StunProbe.kt

@@ -60,6 +60,15 @@ class StunProbe(
 
     override suspend fun run(ctx: Context, ids: ProbeIds): Test = withContext(Dispatchers.IO) {
         val b = TestBuilder(type, tier, ids)
+        // Without a server there is nothing to ask. Skipped rather than failed: "the STUN test
+        // failed" reads as a finding about the network, when the truth is that this device is
+        // not enrolled anywhere and no packet was ever sent.
+        if (serverHost.isBlank()) {
+            return@withContext b.build(
+                TestStatus.SKIPPED,
+                evidence = buildJsonObject { put("reason", "no server configured to ask") },
+            )
+        }
         DatagramSocket().use { sock ->
             sock.soTimeout = 3000
             val localPort = sock.localPort