app: scaffold echolot-app + core-protocol — client spine verified live vs fmr

Multi-module Android app, built bottom-up from a verifiable core.
core-protocol is pure Kotlin/JVM (no Android SDK): SPKI-pinned control
plane (enroll/profile/session over HttpsURLConnection — API-1 compatible,
hostname verification off, trust is the pin), HKDF-SHA256 session keys,
ELT1 UDP data plane (HMAC gate, ECHO+observation, MTU probe) —
byte-compatible with the Go server.

Unit tests incl. the RFC 5869 HKDF vector (key derivation provably matches
the server). LiveServerTest + scripts/test-fmr.sh prove the client
end-to-end against the deployed fmr server: profile (8 caps), session,
ECHO rtt~11ms with the observation block returning our observed NAT port,
MTU 1400->1400, observations. Live test self-skips without ECHOLOT_LIVE_*.

Two client bugs caught live: java.net.http hostname verification (→
HttpsURLConnection, also the Android-minSdk-26 choice) and ECHO padding
needed for the observation to survive anti-amplification.

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

echolot-app/gradle/libs.versions.toml

@@ -0,0 +1,10 @@
+[versions]
+kotlin = "2.2.10"
+kotlinxSerialization = "1.7.3"
+
+[libraries]
+kotlinx-serialization-json = { group = "org.jetbrains.kotlinx", name = "kotlinx-serialization-json", version.ref = "kotlinxSerialization" }
+
+[plugins]
+kotlin-jvm = { id = "org.jetbrains.kotlin.jvm", version.ref = "kotlin" }
+kotlin-serialization = { id = "org.jetbrains.kotlin.plugin.serialization", version.ref = "kotlin" }