test(arch): keep Room and query strings inside the data layer

The seam is only worth having if something checks it. This fails the
build if androidx.room, an entity or a query string appears outside
data/, rather than leaving it to review to notice.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

app/src/test/java/de/jeanlucmakiola/clockula/ArchitectureRulesTest.kt

@@ -0,0 +1,69 @@
+package de.jeanlucmakiola.clockula
+
+import com.google.common.truth.Truth.assertThat
+import org.junit.jupiter.api.Test
+import java.io.File
+
+/**
+ * docs/PLAN.md §0's boundary, made mechanical: the app talks to repository
+ * interfaces and plain-Kotlin models, and only `data/` knows Room exists.
+ * A grep is a weaker tool than a compiler, but it is the one that fails the
+ * build when someone reaches through the seam.
+ */
+class ArchitectureRulesTest {
+
+    private val mainSourceRoot: File = listOf(File("src/main/java"), File("app/src/main/java"))
+        .firstOrNull { it.isDirectory }
+        ?: error("Could not resolve the main source root from ${File("").absolutePath}")
+
+    private val kotlinSources: List<File> =
+        mainSourceRoot.walkTopDown().filter { it.isFile && it.extension == "kt" }.toList()
+
+    private val outsideDataLayer: List<File> =
+        kotlinSources.filterNot { it.invariantSeparatorsPath.contains("de/jeanlucmakiola/clockula/data/") }
+
+    private fun List<File>.containing(vararg needles: String): List<String> =
+        filter { file -> needles.any { it in file.readText() } }.map { it.invariantSeparatorsPath }
+
+    @Test
+    fun `Room does not leak above the data layer`() {
+        val offenders = outsideDataLayer.containing("androidx.room")
+
+        assertThat(offenders).isEmpty()
+    }
+
+    @Test
+    fun `no Dao or Query annotation appears outside the data layer`() {
+        val offenders = outsideDataLayer.containing("@Dao", "@Query")
+
+        assertThat(offenders).isEmpty()
+    }
+
+    @Test
+    fun `no entity or dao type is named outside the data layer`() {
+        val offenders = outsideDataLayer.containing(
+            "AlarmEntity",
+            "TimerEntity",
+            "WorldClockEntity",
+            "LapEntity",
+            "AlarmDao",
+            "TimerDao",
+            "WorldClockDao",
+            "LapDao",
+            "ClockulaDatabase",
+        )
+
+        assertThat(offenders).isEmpty()
+    }
+
+    @Test
+    fun `the domain is free of Android types`() {
+        val domain = kotlinSources
+            .filter { it.invariantSeparatorsPath.contains("de/jeanlucmakiola/clockula/domain/") }
+        val offenders = domain.filter { file ->
+            file.readLines().any { it.startsWith("import android.") || it.startsWith("import androidx.") }
+        }.map { it.invariantSeparatorsPath }
+
+        assertThat(offenders).isEmpty()
+    }
+}