app: net.captive_portal probe — reproduce Android's internet/portal checks

Mirrors NetworkMonitor: per active network, fetch the AOSP default
generate_204 endpoints and check for HTTP 204 No Content.
- HTTPS https://www.google.com/generate_204 == 204 -> validated internet
- HTTP http://connectivitycheck.gstatic.com/generate_204: 204 -> clean;
  an unfollowed 3xx or a 200-with-body -> captive portal (Location captured)
- both fail -> no_internet
Per-network verdicts (bound via Network.openConnection), redirects not
followed (the 3xx IS the evidence). Findings: captive_portal (medium) and
no_internet (high). New test type net.captive_portal (net family ->
connectivity category). App gains usesCleartextTraffic (a network
diagnostic that intentionally probes plain HTTP).

Builds; measurement verdict tests still green. On-device verification
deferred with the rest (flaky test devices).

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

echolot-app/core-measurement/src/main/kotlin/app/echo_lot/measurement/Test.kt

@@ -55,6 +55,8 @@ object TestType {
     const val LINK_SNAPSHOT = "link.snapshot"
     const val LINK_DHCP_RENEWAL_WATCH = "link.dhcp_renewal_watch"
     const val LINK_IP_MONITOR = "link.ip_monitor"
+    // net — connectivity validation (reproduces Android's NetworkMonitor generate_204 checks)
+    const val NET_CAPTIVE_PORTAL = "net.captive_portal"
     // icmp
     const val ICMP_PING4 = "icmp.ping4"
     const val ICMP_PING6 = "icmp.ping6"
@@ -131,7 +133,7 @@ object TestType {
 
     /** Maps a dotted test type to its §7.2 category for verdict rollup. */
     fun category(type: String): Category = when (type.substringBefore('.')) {
-        "link", "icmp", "trace", "traceroute", "train", "port", "time" -> Category.CONNECTIVITY
+        "link", "icmp", "trace", "traceroute", "train", "port", "time", "net" -> Category.CONNECTIVITY
         "dns" -> Category.DNS
         "nat" -> Category.NAT
         "mtu" -> Category.MTU