fix: critical bug in PiezoPacketCollector processEndPacket

CRITICAL: In multi-channel mode, "ree:" packets between channels
were silently dropped instead of saving the current channel data.
This caused channel data loss when channels are separated by "ree:".

Fixed: Added missing isMultiChannel + ree: branch that calls
saveCurrentChannel() — matching Swift behavior exactly.

Found via line-by-line Swift vs Kotlin verification.

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

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

@@ -130,12 +130,19 @@ class PiezoPacketCollector {
 
     private fun processEndPacket(prefix: String, bytes: ByteArray) {
         if (isMultiChannel && prefix == "raa:") {
+            // Final completion marker — save last channel and finish
             if (hasFirstPacket && rawData.isNotEmpty()) saveCurrentChannel()
             isComplete = true
             val validChannels = channelResults.filterNotNull()
             onLog?.invoke("raa: multi-channel complete — ${validChannels.size} channels")
             onMultiChannelComplete?.invoke(validChannels)
-        } else if (!isMultiChannel) {
+        } else if (isMultiChannel) {
+            // ree: in multi-channel = channel separator, save and continue
+            if (hasFirstPacket && rawData.isNotEmpty()) {
+                saveCurrentChannel()
+            }
+        } else {
+            // Single-channel complete (ree:)
             finishSingleChannel()
         }
     }