android: cloud module scaffold (retrofit, opt-in, disabled by default)

android/app/proguard-rules.pro

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+# MAG160C release (R8) keep rules.
+
+# Cloud scaffold (Phase D): the Retrofit interface + DTOs are only reached via
+# reflection (Retrofit proxies, Gson field mapping), so keep them intact.
+-keep class com.mag160c.thermal.cloud.** { *; }
+-keepattributes Signature
+-keepattributes *Annotation*
+
+# Gson's reflective type adapters for the cloud DTOs.
+-keep class com.google.gson.** { *; }
+-dontwarn com.google.gson.**
+-dontwarn okhttp3.**
+-dontwarn okio.**
+-dontwarn retrofit2.**