Implement rotation-clone presence engine (#1) + unit tests + CI tests

- detect/PresenceEngine.kt: identity-agnostic streaming detector for key-rotating clones (novel-churn gate, seamless-handover chain, co-movement gate, coherence). Full design in docs/detection-rotation-clone.md.
- Wired into ScanService as a parallel signal; CONFIRMED verdict raises a distinct rotating-tracker alert (softer tiers stay silent pending real-capture validation).
- PresenceEngineTest: synthetic clone (moving -> CONFIRMED), stationary clone (gated), single real tracker + far-ambient (CLEAR).
- CI now runs :app:testDebugUnitTest so the engine has behavioural verification, not just a compile check.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0187UiEtasyowhEBYs6s9iF5

gradle/libs.versions.toml

@@ -9,6 +9,7 @@ composeBom = "2024.12.01"
 material3 = "1.3.1"
 playServicesLocation = "21.3.0"
 room = "2.6.1"
+junit = "4.13.2"
 
 [libraries]
 androidx-core-ktx = { group = "androidx.core", name = "core-ktx", version.ref = "coreKtx" }
@@ -26,6 +27,7 @@ play-services-location = { group = "com.google.android.gms", name = "play-servic
 androidx-room-runtime = { group = "androidx.room", name = "room-runtime", version.ref = "room" }
 androidx-room-ktx = { group = "androidx.room", name = "room-ktx", version.ref = "room" }
 androidx-room-compiler = { group = "androidx.room", name = "room-compiler", version.ref = "room" }
+junit = { group = "junit", name = "junit", version.ref = "junit" }
 
 [plugins]
 android-application = { id = "com.android.application", version.ref = "agp" }