test: characterize getTasksForResidence filter contract

Locks down the contract that becomes the primary path for residence
detail in Phase 3:
- filters _allTasks by residenceId
- returns empty shell for residence with no tasks (vs null for cache miss)
- returns null when _allTasks itself is null (caller must hit API)

composeApp/src/commonTest/kotlin/com/tt/honeyDue/data/DataManagerTaskCacheTest.kt

@@ -88,6 +88,53 @@ class DataManagerTaskCacheTest {
         assertEquals("v2", inProgress.tasks.first { it.id == 5 }.title)
     }
 
+    /// Characterization: getTasksForResidence filters _allTasks by
+    /// residence id. This is the helper that becomes the primary path
+    /// for residence-detail in Phase 3 (collapse the dual cache).
+    @Test
+    fun getTasksForResidence_filtersAllTasksByResidenceId() {
+        // Seed _allTasks with tasks across two residences via the upsert path.
+        DataManager.updateTask(sampleTask(id = 1, residenceId = 100, column = "upcoming_tasks"))
+        DataManager.updateTask(sampleTask(id = 2, residenceId = 100, column = "overdue_tasks"))
+        DataManager.updateTask(sampleTask(id = 3, residenceId = 200, column = "upcoming_tasks"))
+
+        val r100 = DataManager.getTasksForResidence(100)
+        assertNotNull(r100)
+        val r100Ids = r100.columns.flatMap { it.tasks }.map { it.id }.toSet()
+        assertEquals(setOf(1, 2), r100Ids)
+
+        val r200 = DataManager.getTasksForResidence(200)
+        assertNotNull(r200)
+        val r200Ids = r200.columns.flatMap { it.tasks }.map { it.id }.toSet()
+        assertEquals(setOf(3), r200Ids)
+
+        // Counts on each column must match the filtered task lists.
+        for (column in r100.columns) {
+            assertEquals(column.tasks.size, column.count, "column ${column.name} count mismatch")
+        }
+    }
+
+    /// Characterization: residenceId with no tasks returns a non-null
+    /// shell so the residence-detail screen can distinguish "loading"
+    /// (null) from "loaded, no tasks" (non-null with empty columns).
+    @Test
+    fun getTasksForResidence_returnsEmptyShellForResidenceWithNoTasks() {
+        DataManager.updateTask(sampleTask(id = 1, residenceId = 100, column = "upcoming_tasks"))
+
+        val r999 = DataManager.getTasksForResidence(999)
+        assertNotNull(r999, "residence with no tasks must return an empty shell, not null")
+        assertEquals(0, r999.columns.sumOf { it.tasks.size })
+    }
+
+    /// Characterization: when _allTasks is null entirely (cache never
+    /// populated), getTasksForResidence returns null — caller must call
+    /// the API path. Phase 3's getTasksByResidence relies on this.
+    @Test
+    fun getTasksForResidence_returnsNullWhenAllTasksIsNull() {
+        DataManager.clear()
+        assertEquals(null, DataManager.getTasksForResidence(100))
+    }
+
     /// Lockdown: updateTask must NOT touch `_tasksByResidence`. That cache
     /// is being deleted in Phase 3; until then, updateTask must leave it
     /// alone. If a future commit re-introduces the conditional write