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

.github/workflows/build.yml

@@ -26,9 +26,13 @@ jobs:
       - name: Set up Android SDK
         uses: android-actions/setup-android@v3
 
-      - name: Build debug APK
+      - name: Run unit tests
         run: |
           chmod +x ./gradlew
+          ./gradlew :app:testDebugUnitTest --stacktrace --no-daemon
+
+      - name: Build debug APK
+        run: |
           ./gradlew :app:assembleDebug --stacktrace --no-daemon
 
       - name: Stage APK