data: add ColumnReader abstraction + Cursor.toCalendarSource mapper

Deviation from Plan 02: the JVM mockable-android.jar stubs every Cursor
method even with isReturnDefaultValues=true (returns null/0 regardless of
the underlying MatrixCursor backing). Introduce an internal ColumnReader
interface so mappers stay pure-Kotlin and JVM-testable via MapColumnReader,
while production reads through CursorColumnReader.

app/src/test/java/de/jeanlucmakiola/calendula/data/calendar/CalendarMapperTest.kt

@@ -0,0 +1,68 @@
+package de.jeanlucmakiola.calendula.data.calendar
+
+import com.google.common.truth.Truth.assertThat
+import org.junit.jupiter.api.Test
+
+class CalendarMapperTest {
+
+    private fun reader(
+        id: Long = 1L,
+        displayName: String? = "Cal",
+        accountName: String? = "x@y",
+        accountType: String? = "LOCAL",
+        color: Int = 0,
+        visible: Int = 1,
+    ): MapColumnReader = MapColumnReader(
+        CalendarProjection.IDX_ID to id,
+        CalendarProjection.IDX_DISPLAY_NAME to displayName,
+        CalendarProjection.IDX_ACCOUNT_NAME to accountName,
+        CalendarProjection.IDX_ACCOUNT_TYPE to accountType,
+        CalendarProjection.IDX_COLOR to color,
+        CalendarProjection.IDX_VISIBLE to visible,
+    )
+
+    @Test
+    fun `happy path maps all six columns`() {
+        val src = reader(
+            id = 42L,
+            displayName = "Work",
+            accountName = "x@y",
+            accountType = "com.google",
+            color = 0xFF112233.toInt(),
+            visible = 1,
+        ).toCalendarSource()
+        assertThat(src).isEqualTo(
+            de.jeanlucmakiola.calendula.domain.CalendarSource(
+                id = 42L,
+                displayName = "Work",
+                accountName = "x@y",
+                accountType = "com.google",
+                color = 0xFF112233.toInt(),
+                isVisibleInSystem = true,
+            )
+        )
+    }
+
+    @Test
+    fun `null displayName falls back to placeholder`() {
+        val src = reader(displayName = null).toCalendarSource()
+        assertThat(src.displayName).isEqualTo(Fallbacks.UNNAMED_CALENDAR)
+    }
+
+    @Test
+    fun `visible flag 0 maps to false`() {
+        assertThat(reader(visible = 0).toCalendarSource().isVisibleInSystem).isFalse()
+    }
+
+    @Test
+    fun `visible flag 1 maps to true`() {
+        assertThat(reader(visible = 1).toCalendarSource().isVisibleInSystem).isTrue()
+    }
+
+    @Test
+    fun `null accountName and accountType coerce to empty string`() {
+        val src = reader(accountName = null, accountType = null).toCalendarSource()
+        assertThat(src.accountName).isEqualTo("")
+        assertThat(src.accountType).isEqualTo("")
+    }
+}

app/src/test/java/de/jeanlucmakiola/calendula/data/calendar/MapColumnReader.kt

@@ -0,0 +1,29 @@
+package de.jeanlucmakiola.calendula.data.calendar
+
+/**
+ * Test-only ColumnReader. Backed by a Map<Int, Any?>; any missing index is
+ * treated as null. Numeric getters coerce via toLong/toInt; non-numeric values
+ * yield zero (matching Android Cursor behavior for type-mismatched reads).
+ */
+internal class MapColumnReader(values: Map<Int, Any?>) : ColumnReader {
+
+    private val data: Map<Int, Any?> = values
+
+    constructor(vararg pairs: Pair<Int, Any?>) : this(pairs.toMap())
+
+    override fun getLong(index: Int): Long = when (val v = data[index]) {
+        is Number -> v.toLong()
+        is String -> v.toLongOrNull() ?: 0L
+        else -> 0L
+    }
+
+    override fun getString(index: Int): String? = data[index]?.toString()
+
+    override fun getInt(index: Int): Int = when (val v = data[index]) {
+        is Number -> v.toInt()
+        is String -> v.toIntOrNull() ?: 0
+        else -> 0
+    }
+
+    override fun isNull(index: Int): Boolean = data[index] == null
+}