fix: Placement 오탐 개선 — MIN_URINE_LEN 12, 미감지 채널 벽 표시 제거

- MIN_URINE_LEN_PLACEMENT 10 → 12 (짧은 오탐 필터 강화)
- 미감지 채널(channelDetected=false)은 그래프에서 ant/post 선, depth mm, cyan 영역 미표시
- 파형 자체는 유지 (신호 확인용)

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

app/src/main/java/com/example/medilightv2android/ui/views/monitoring/PlacementGuideView.kt

@@ -256,8 +256,11 @@ fun PlacementGuideView(appState: AppState) {
                     ) {
                         Text("Echo Waveforms", fontSize = 13.sp, fontWeight = FontWeight.Bold, color = MlPrimary)
                         for (chData in lastChannelData.filter { it.buffer.isNotEmpty() }) {
-                            val chAnalysis = analyzer.analyzeChannel(chData.buffer, chData.channel)
                             val detected = if (chData.channel < channelDetected.size) channelDetected[chData.channel] else false
+                            val showWalls = if (detected) {
+                                val chAnalysis = analyzer.analyzeChannel(chData.buffer, chData.channel)
+                                chAnalysis.result
+                            } else null
                             Column(verticalArrangement = Arrangement.spacedBy(2.dp)) {
                                 Row(verticalAlignment = Alignment.CenterVertically) {
                                     Box(
@@ -266,11 +269,10 @@ fun PlacementGuideView(appState: AppState) {
                                     )
                                     Spacer(modifier = Modifier.width(4.dp))
                                     Text("CH${chData.channel}", fontSize = 11.sp, fontWeight = FontWeight.Bold)
-                                    if (chAnalysis.result != null) {
-                                        val r = chAnalysis.result
+                                    if (showWalls != null) {
                                         Spacer(modifier = Modifier.width(6.dp))
                                         Text(
-                                            "${"%.0f".format(r.antDepthMm)}–${"%.0f".format(r.postDepthMm)}mm",
+                                            "${"%.0f".format(showWalls.antDepthMm)}–${"%.0f".format(showWalls.postDepthMm)}mm",
                                             fontSize = 9.sp, color = Color.Cyan
                                         )
                                     }
@@ -279,8 +281,8 @@ fun PlacementGuideView(appState: AppState) {
                                     buffer = chData.buffer,
                                     peakIndex = chData.peakIndex.toInt(),
                                     analyzer = analyzer,
-                                    antIndex = chAnalysis.result?.ant,
-                                    postIndex = chAnalysis.result?.post,
+                                    antIndex = showWalls?.ant,
+                                    postIndex = showWalls?.post,
                                     modifier = Modifier.fillMaxWidth().height(80.dp)
                                 )
                             }
@@ -307,7 +309,7 @@ fun PlacementGuideView(appState: AppState) {
                     val urineLens = MutableList(6) { 0 }
                     val allWalls: MutableList<Pair<Int, Int>?> = MutableList(6) { null }
 
-                    val MIN_URINE_LEN_PLACEMENT = 10 // Placement에서는 최소 10칸 이상이어야 유효
+                    val MIN_URINE_LEN_PLACEMENT = 12
 
                     for (ch in channels) {
                         if (ch.buffer.isNotEmpty() && ch.channel < 6) {