feat: developer mode, SWEEP placement, gradient guide, spot flicker fix

- Developer mode: 캐릭터 3탭 토글, dev-only settings/graphs/logs
- SWEEP placement (CKLaw): BladderSphereSeek circle fit + RTracker
  + argmax + coronal lateral + R-peak gate GREEN lock
- Gradient placement: weighted center gradient 기반 상하 판정,
  방향 로직 수정 (ch0=위, ch3=아래), 초기 멘트 "치골 위에 배치"
- Spot 측정: isSpotInProgress 플래그로 중간값 화면 깜빡임 제거
- 좌우 애매한 ↔ 멘트 제거 → 구체적 방향 또는 상태 안내

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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

@@ -24,6 +24,7 @@ class AppState(private val context: Context) {
     var bladderLevel by mutableStateOf(BladderLevel())
     var isDeviceConnected by mutableStateOf(false)
     var isDemoMode by mutableStateOf(false)
+    var isDevMode by mutableStateOf(false)
     var piezoSettings by mutableStateOf(PiezoSettings())
     var catheterCount = mutableIntStateOf(0)