fix: critical code review — thread safety, state management

CRITICAL fixes:
- #1: lastMultiChannelResult 변경을 handler.post로 (메인 스레드 보장)
- #3: measureDoneSignal var=remember → val mutableStateOf (recomposition 안전)
- #34,35: BleManager bluetoothGatt/tx/rxCharacteristic/lastRxTimestamp @Volatile
- #66: GreenZoneConstants 모든 mutable var @Volatile
- #56: PiezoHW.distancePerSample @Volatile
- nested handler.post 제거 (불필요한 프레임 지연)
- maa throttle deadlock 방지 (Boolean 반환 + state 리셋)
- Plot/Log 불일치 수정 (channelLogs를 allWalls 기준으로 통일)
- Placement 센서 위치/화살표 조정, gradient dead zone, CV relaxation
- 개발자 모드 CV 표시, 가이드 상자 항상 표시

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

app/src/main/java/com/example/medilightv2android/ble/BleManager.kt

@@ -48,9 +48,9 @@ class BleManager private constructor(private val context: Context) {
     private val bluetoothManager = context.getSystemService(Context.BLUETOOTH_SERVICE) as BluetoothManager
     private val bluetoothAdapter: BluetoothAdapter? = bluetoothManager.adapter
     private var bluetoothLeScanner: BluetoothLeScanner? = null
-    private var bluetoothGatt: BluetoothGatt? = null
-    private var txCharacteristic: BluetoothGattCharacteristic? = null
-    private var rxCharacteristic: BluetoothGattCharacteristic? = null
+    @Volatile private var bluetoothGatt: BluetoothGatt? = null
+    @Volatile private var txCharacteristic: BluetoothGattCharacteristic? = null
+    @Volatile private var rxCharacteristic: BluetoothGattCharacteristic? = null
     private val handler = Handler(Looper.getMainLooper())
     val debugLogger = com.example.medilightv2android.services.BleDebugLogger.getInstance(context)
     private var scanTimer: Runnable? = null
@@ -62,7 +62,7 @@ class BleManager private constructor(private val context: Context) {
     private var reconnectAttempt = 0
     private val reconnectIntervalMs: Long = 3000
     private var reconnectTimer: Runnable? = null
-    private var lastRxTimestamp: Long = 0
+    @Volatile private var lastRxTimestamp: Long = 0
     private var watchdogTimer: Runnable? = null
     private val watchdogTimeoutMs: Long = 15000
 
@@ -238,6 +238,7 @@ class BleManager private constructor(private val context: Context) {
         scheduleAutoReconnect()
     }
 
+
     fun disconnect() {
         isUserDisconnect = true
         cancelAutoReconnect()
@@ -395,16 +396,16 @@ class BleManager private constructor(private val context: Context) {
         sendRaw(CRC16.buildCommandBE("mec", intArrayOf(freqOption, delayUs, 140, cycles, 1, piezoCh)))
     }
 
-    fun sendAllChannels(mode: Int = 0) {
+    fun sendAllChannels(mode: Int = 0): Boolean {
         val now = System.currentTimeMillis()
-        if (now - lastMaaSentMs < 1000) {
+        if (now - lastMaaSentMs < 800) {
             android.util.Log.w("BleManager", "maa THROTTLED — ${now - lastMaaSentMs}ms since last")
-            debugLogger.warn("maa THROTTLED ${now - lastMaaSentMs}ms")
-            return
+            return false
         }
         lastMaaSentMs = now
         piezoCollector.startMultiChannel(6)
         sendRaw(CRC16.buildCommandASCII("maa", " "))
+        return true
     }
 
     // ── NIRS Commands ──
@@ -453,18 +454,18 @@ class BleManager private constructor(private val context: Context) {
         sendRaw(CRC16.buildCommandASCII("mbb", " "))
     }
 
-    /** 6채널 측정만 (maa) — 최소 1초 간격 throttle */
+    /** 6채널 측정만 (maa) — 최소 800ms 간격 throttle */
     private var lastMaaSentMs = 0L
-    fun sendChannelsOnly() {
+    fun sendChannelsOnly(): Boolean {
         val now = System.currentTimeMillis()
-        if (now - lastMaaSentMs < 1000) {
+        if (now - lastMaaSentMs < 800) {
             android.util.Log.w("BleManager", "maa THROTTLED — ${now - lastMaaSentMs}ms since last")
-            debugLogger.warn("maa THROTTLED ${now - lastMaaSentMs}ms")
-            return
+            return false
         }
         lastMaaSentMs = now
         piezoCollector.startMultiChannel(6)
         sendRaw(CRC16.buildCommandASCII("maa", " "))
+        return true
     }
 
     fun startBatteryPolling() {