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/main/java/de/jeanlucmakiola/calendula/data/calendar/CalendarMapper.kt

@@ -0,0 +1,13 @@
+package de.jeanlucmakiola.calendula.data.calendar
+
+import de.jeanlucmakiola.calendula.domain.CalendarSource
+
+internal fun ColumnReader.toCalendarSource(): CalendarSource = CalendarSource(
+    id = getLong(CalendarProjection.IDX_ID),
+    displayName = getString(CalendarProjection.IDX_DISPLAY_NAME)
+        ?: Fallbacks.UNNAMED_CALENDAR,
+    accountName = getString(CalendarProjection.IDX_ACCOUNT_NAME).orEmpty(),
+    accountType = getString(CalendarProjection.IDX_ACCOUNT_TYPE).orEmpty(),
+    color = getInt(CalendarProjection.IDX_COLOR),
+    isVisibleInSystem = getInt(CalendarProjection.IDX_VISIBLE) != 0,
+)

app/src/main/java/de/jeanlucmakiola/calendula/data/calendar/ColumnReader.kt

@@ -0,0 +1,22 @@
+package de.jeanlucmakiola.calendula.data.calendar
+
+import android.database.Cursor
+
+/**
+ * Read-only view over a single row's columns by index. Lets the mappers work
+ * on pure-Kotlin test fixtures (MapColumnReader) on the JVM, while the
+ * production path adapts an Android Cursor row via CursorColumnReader.
+ */
+internal interface ColumnReader {
+    fun getLong(index: Int): Long
+    fun getString(index: Int): String?
+    fun getInt(index: Int): Int
+    fun isNull(index: Int): Boolean
+}
+
+internal class CursorColumnReader(private val cursor: Cursor) : ColumnReader {
+    override fun getLong(index: Int): Long = cursor.getLong(index)
+    override fun getString(index: Int): String? = cursor.getString(index)
+    override fun getInt(index: Int): Int = cursor.getInt(index)
+    override fun isNull(index: Int): Boolean = cursor.isNull(index)
+}