feat: PlacementGuideView + MeasurementLogService + navigation flow

PlacementGuideView.kt:
- 6-sensor Canvas layout (CH0-CH3 vertical + CH4/CH5 lateral wings)
- Per-channel UrinAI detection (green/red color coding)
- Direction guidance (up/down/left/right)
- Check Position → maa → detect → display
- Start Measurement when 4+ channels detected

MeasurementLogService.kt:
- JSON Lines format, daily rotation
- Events: measurement, placement, ble_connected/disconnected, error
- 90-day auto-purge
- App internal storage: vesiscan_logs/YYYY-MM-DD.jsonl

AppState.kt + MainActivity.kt:
- PLACEMENT_GUIDE screen added to navigation
- Flow: personalization → placementGuide → piezoMonitoring

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/src/main/java/com/example/medilightv2android/AppState.kt

@@ -12,7 +12,7 @@ import com.example.medilightv2android.services.*
 
 enum class AppScreen {
     ONBOARDING, CONSENT, REGISTRATION, HOME, SENSOR_SELECT, DEVICE_SCAN,
-    NIRS_CALIBRATION, PIEZO_PERSONALIZATION, PIN_SETUP, PIN_ENTRY,
+    NIRS_CALIBRATION, PIEZO_PERSONALIZATION, PLACEMENT_GUIDE, PIN_SETUP, PIN_ENTRY,
     VIVAMYO_MONITORING, PIEZO_MONITORING, MEASUREMENT_HISTORY,
     VOIDING_DIARY, URINE_CAMERA, REMINDER_SETTINGS
 }
@@ -121,6 +121,10 @@ class AppState(private val context: Context) {
 
     fun personalizationComplete() {
         storage.piezoSettings = piezoSettings
+        currentScreen = AppScreen.PLACEMENT_GUIDE
+    }
+
+    fun placementComplete() {
         currentScreen = AppScreen.PIEZO_MONITORING
     }