refactor: Replace Moshi with kotlinx.serialization

Remove Moshi dependency entirely, completing the migration to kotlinx.serialization. All JSON serialization now uses kotlinx with explicit @SerialName annotations for wire format stability.

- Migrate PodDeviceCache from Moshi to kotlinx Json injection

- Add MapIntByteArrayBase64Serializer for BleScanResult cache compat

- Strip @JsonClass/@Json annotations from all dual-annotated classes

- Delete Moshi adapters, ProGuard rules, and build config

- Convert compat tests to pure kotlinx round-trip tests

app/src/test/java/testhelpers/json/JsonExtensions.kt

@@ -1,29 +0,0 @@
-package testhelpers.json
-
-import com.squareup.moshi.JsonReader
-import com.squareup.moshi.Moshi
-import okio.Buffer
-import okio.ByteString.Companion.encode
-import okio.buffer
-import okio.sink
-import java.io.File
-
-
-fun String.toComparableJson(): String {
-    val value = Buffer().use {
-        it.writeUtf8(this)
-        val reader = JsonReader.of(it)
-        reader.readJsonValue()
-    }
-
-    val adapter = Moshi.Builder().build().adapter(Any::class.java).indent("    ")
-
-    return adapter.toJson(value)
-}
-
-fun String.writeToFile(file: File) = encode().let { text ->
-    require(!file.exists())
-    file.parentFile?.mkdirs()
-    file.createNewFile()
-    file.sink().buffer().use { it.write(text) }
-}